Algorithm and Data Structure Summary

algorithm, data-structure, python, swift

算法与数据结构汇总

Big-O Concept

{O 上界} {Ω Omega 下界} {Θ Theta 上下界}
big_o_complexity

Sorting Algorithm Complexity

sort_algorithm_complexity
sorting_algo_gif (credit to: https://www.toptal.com/developers/sorting-algorithms/)

Data Structure 数据结构

array 数组

  • 特性
    • ordered
    • random-access (constant O(1) time access)
    • get_length - O(1)
    • append_last - O(1)

dictionary 字典

key-value hashable

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# python
scores = {"Eric": 90, "Mark": 80, "Wayne": 60}

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// swift
var scores: [String: Int] = ["Eric": 90, "Mark": 80, "Wayne": 60]

linked list 链表

benefit: can add and remove items from the beginning of the list in constant time.

singly linked list 单向链表, doubly linked list 双向链表

  • runer technique: iterate through the linked list with two pointers simultaneously (fast and slow) 
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# python
# singly linked list 单向链表
class SinglyLinkedListNode:
    def __init__(self, value, next=None):
        self.val = value
        self.next = next

# doubly linked list 双向链表
class DoublyLinkedListNode:
    def __init__(self, value, next=None, prev=None):
        self.val = value
        self.next = next
        self.prev = prev
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// swift
// singly linked list 单向链表
public class SinglyLinkedListNode<Value> {
    public var value: Value
    public var next: SinglyLinkedListNode?

    public init(value: Value, next: SinglyLinkedListNode?=nil) {
        self.value = value
        self.next = next
    }
}

// doubly linked list 双向链表
public class DoublyLinkedListNode<Value> {
    public var value: Value
    public var next: DoublyLinkedListNode?
    public var prev: DoublyLinkedListNode?

    public init(value: Value, next: DoublyLinkedListNode?=nil, prev: DoublyLinkedListNode?=nil) {
        self.value = value
        self.next =next
        self.prev = prev
    }
}

set 集合

unordered collecition of unique values

  • dic 字典
  • hashtable
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# python
my_set = set([1,2])    # + - & | ^
empty_set = set()
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// swift
var my_set: Set<Int> = [1, 2]
var my_set2 = Set<Int>()

list 列表

stack queue priority queue heap

stack 栈

{LIFO} {push, pop, top, isEmpty}

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# python
stack = [1, 2, 3]
stack.append(4)
num = stack.pop()   # >>> 4
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// swift
public struct Stack<E> {
    private var stack: [E] = []
    public mutating func push(_ element: E) {
        stack.append(element)
    }
    public mutating func pop() -> E? {
        return stack.popLast()
    }
}

queue 队列

{FIFO} {enqueue, dequeue, front, rear, isEmpty}

  • can be created in four ways:
    • array - {dequeue in this struct takes O(n)}
    • doubly linked list - {elements aren’t in contiguous blocks of memory. will increase access time}
    • ring buffer - {fixed size}
    • two stacks - {best}
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# python3
# double ended queue
# deque 在首尾两端快速插入和删除而设计
from collections import deque
queue  = deque([1, 2, 3])
queue.append(4)
queue.popleft()
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// swift
public struct QueueStack<T>: Queue {
    private var leftStack: [T] = []
    private var rightStack: [T] = []
    public init() { }

    public var isEmpty: Bool {
        return leftStack.isEmpty && rightStack.isEmpty
    }

    public var peek: T? {
        return !leftStack.isEmpty ? leftStack.last : rightStack.first
    }

    public mutating func enqueue(_ element: T) -> Bool {
        rightStack.append(element)
        return true
    }

    public mutating func dequeue() -> T? {
        if leftStack.isEmpty {
            leftStack = rightStack.reversed()
            rightStack.removeAll()
        }
        return leftStack.popLast()
    }
}
priority queue 优先队列
heap 堆

can also be developed in tree structure
[]

graph 图

{G = <V-vertex, E-edge>} {directed, undirected ,weighted, unweighted}

  • complete graph 完全图
  • dense graph 稠密图
  • spares graph 稀疏图
  • {表示方法: adjacency matrix邻接矩阵 - 适合稠密图 & adjacency lists邻接链表 - 适合稀疏图} 
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# python
class AdjacencyList:
    def __init__(self):
        self.a_list = {}

    def addEdge(self, from_vertex, to_vertex):
        if from_vertex in self.a_list.keys():
            self.a_list[from_vertex].append(to_vertex)
        else:
            self.a_list[from_vertex] = [to_vertex]

    def print_a_list(self):
        for from_v in self.a_list:
            print('{vertex} ->'.format(vertex = from_v), ' -> '.join(
                  [str(to_v) for to_v in self.a_list[from_v]]))

tree 树

{|E| = |V| - 1}

  • tree
  • binary tree
  • binary search tree (BST) 二叉查找树 {math.floor(logn) <= h <= n-1}
  • balanced search tree 平衡查找树
    • self-balancing 自平衡查找树
      • AVL tree
        • 每个节点的左右子树高度差不超过1
      • red-black tree 红黑树
        • 能容忍同一节点的一棵子树的高度是另一棵子树的两倍
      • splay tree 分裂树
    • 允许单个节点中包含不只一个元素
      • 2-3 tree
      • 2-3-4 tree
      • B tree
  • complete binary tree 完全二叉树
  • heap (binary heaps) {complete binary tree}
    • 可以用完全二叉树实现, 树的每一层都是满的，除了最后一层最右边元素可能缺位
    • 父母优势, 每一个节点的键都要大于等于它子女的键 
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     10
   /    \      if using array:
  8      7          parents  leaves
 / \    / \     0   1  2  3| 4  5  6
5   2  6       [ , 10, 8, 7, 5, 2, 6]
  • Trie tree (prefix trees) 单词查找树
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        root
       /    \
      T      C
     / \    / \
    R.  O. A.  O.

tree

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# python
class TreeNode:
    def __init__(self, value):
        self.val = value
        self.children = []
    def add(child):
        children.append(child)
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// swift
public class TreeNode<T> {
    public var value: T
    public var children: [TreeNode] = []
    public init(_ value: T) {
        self.value = value
    }
    public func add(_ child: TreeNode) {
        children.append(child)
    }
}

binary tree 二叉树

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# python
class BinaryTreeNode:
    def __init__(self, value):
        self.val = value
        self.left = None
        self.right = None
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// swift
public class BinaryTreeNode<Element> {
    public var val: Element
    public var left: BinaryTreeNode?
    public var right: BinaryTreeNode?

    public init(value: Element) {
        self.val = value
    }
}
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// swift
// draw the tree
extension BinaryTreeNode: CustomStringConvertible {
    public var description: String {
        return diagram(for: self)
    }

    private func diagram(for node: BinaryTreeNode?,
                         _ top: String = "",
                         _ root: String = "",
                         _ bottom: String = "") -> String {
        guard let node = node else {
            return root + "nil\n"
        }
        if node.left == nil && node.right == nil {
            return root + "\(node.val)\n"
        }
        return diagram(for: node.right, top + " ", top + "┌──", top + "│ ")
            + root + "\(node.val)\n"
            + diagram(for: node.left, bottom + "│ ", bottom + "└──", bottom + " ")
    }
}

Note: This BinaryTreeNode discription algorithm from Optimizing Collections

  • tree traversal
    • preorder (左中右)
    • inorder （中左右)
    • postorder (左右中) 
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def preorder(tree):
    if tree:
        print(tree.val)
        preorder(tree.left)
        preorder(tree.right)
def inorder(tree):
    if tree:
        inorder(tree.left)
        print(tree.val)
        inorder(tree.right)
def postorder(tree):
    if tree:
        postorder(tree.left)
        postorder(tree.right)
        print(tree.val)
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// swift
extension BinaryTreeNode {
    public func preOrder(visit: (Element) -> ()) {
        visit(val)
        left?.preOrder(visit: visit)
        right?.preOrder(visit: visit)
    }
    public func inOrder(visit: (Element) -> ()) {
        left?.inOrder(visit: visit)
        visit(val)
        right?.inOrder(visit: visit)
    }
    public func postOrder(visit: (Element) -> ()) {
        left?.postOrder(visit: visit)
        right?.postOrder(visit: visit)
        visit(val)
    }
}

binary search tree (BST) 二叉查找树

math.floor(logn) <= h <= n-1
fast lookup, addition, and removal operations: O(logn)

  • value of left child less than its parent
  • value of right child greater than or equal to its parent
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# python
class BinaryTreeNode:
    def __init__(self, value):
        self.val = value
        self.left = None
        self.right = None

    def inorder(self, visit):
        if self.left:
            self.left.inorder(visit)
        visit(self.val)
        if self.right:
            self.right.inorder(visit)

    def get_min(self):
        min = self
        while min.left:
            min = min.left
        return min

    def __repr__(self):
        def diagram(node, top="", root="", bottom=""):
            if not node:
                return root + "None\n"
            if (not node.left) and (not node.right):
                return root + str(node.val) + '\n'
            return (diagram(node.right, top + ' ', top + '┌──', top + '│ ')
                   + root + str(node.val) + '\n'
                   + diagram(node.left, bottom + '│ ', bottom + '└──', bottom + ' '))
        return diagram(self)

class BinarySearchTree:
    def __init__(self):
        self.root = None

    def insert(self, val):
        self.root = self.__insert(self.root, val)

    def __insert(self, node, val):
        if not node:
            return BinaryTreeNode(val)
        if val < node.val:
            node.left = self.__insert(node.left, val)
        else:
            node.right = self.__insert(node.right, val)
        return node

    def contain(self, val):
        current = self.root
        while current:
            if current.val == val:
                return True
            if val < current.val:
                current = current.left
            else:
                current = current.right
        return False

    def remove(self, val):
        self.root = self.__remove(self.root, val)

    def __remove(self, node, val):
        if not node:
            return None
        if val == node.val:
            if (not node.left) and (not node.right):
                return None
            if node.right:
                return node.right
            if node.left:
                return node.left
            node.val = node.right.get_min().val
            node.right = self.__remove(node.right, node.val)
        elif val < node.val:
            node.left = self.__remove(node.left, val)
        else:
            node.right = self.__remove(node.right, val)
        return node

    def inorder(self, root, visit):
        if root:
            self.inorder(root.left, visit)
            visit(root.val)
            self.inorder(root.right, visit)

    def __repr__(self):
        return str(self.root)
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// swift
public class BinaryTreeNode<Element> {
    public var val: Element
    public var left: BinaryTreeNode?
    public var right: BinaryTreeNode?

    public init(value: Element) {
        self.val = value
    }

    // get the min node of the BTS
    var min: BinaryTreeNode {
        return left?.min ?? self
    }
}

extension BinaryTreeNode: CustomStringConvertible {
    // ...
    // same as extension in binary tree
}

public struct BinarySearchTree<Element: Comparable> {
    public private(set) var root: BinaryTreeNode<Element>?

    public init() { }
}

extension BinarySearchTree {
    public mutating func insert(_ val: Element) {
        root = insert(from: root, val: val)
    }

    private func insert(from node: BinaryTreeNode<Element>?, val: Element) -> BinaryTreeNode<Element> {
        guard let node = node else {
            return BinaryTreeNode(value: val)
        }
        if val < node.val {
            node.left = insert(from: node.left, val: val)
        } else {
            node.right = insert(from: node.right, val: val)
        }
        return node
    }

    public func contains(_ value: Element) -> Bool {
        var current = root
        while let node = current {
            if node.val == value {
                return true
            }
            if value < node.val {
                current = node.left
            } else {
                current = node.right
            }
        }
        return false
    }

    public mutating func remove(_ value: Element) {
        root = remove(node: root, value: value)
    }

    private func remove(node: BinaryTreeNode<Element>?, value: Element) -> BinaryTreeNode<Element>? {
        guard let node = node else {
            return nil
        }
        if value == node.val {
            if node.left == nil && node.right == nil {
                return nil
            }
            if node.left == nil {
                return node.right
            }
            if node.right == nil {
                return node.left
            }
            node.val = node.right!.min.val
            node.right = remove(node: node.right, value: node.val)
        } else if value < node.val {
            node.left = remove(node: node.left, value: value)
        } else {
            node.right = remove(node: node.right, value: value)
        }
        return node
    }
}

extension BinarySearchTree: CustomStringConvertible {
    public var description: String {
        return root?.description ?? "empty tree"
    }
}

AVL Tree

self-balancing binary search tree

  • Rotations
    • left
    • right-left
    • right
    • left-right
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# python
__author__ = 'Min'
class AVLTreeNode:
    def __init__(self, value):
        self.val = value
        self.left = None
        self.right = None
        self.height = 0

    def get_min(self):
        min = self
        while min.left:
            min = min.left
        return min

    def get_balanceFactor(self):
        return self.get_left_height() - self.get_right_height()

    def get_left_height(self):
        return self.left.height if self.left else -1

    def get_right_height(self):
        return self.right.height if self.right else -1

    def __repr__(self):
        def diagram(node, top="", root="", bottom=""):
            if not node:
                return root + "None\n"
            if (not node.left) and (not node.right):
                return root + str(node.val) + '\n'
            return (diagram(node.right, top + ' ', top + '┌──', top + '│ ')
                   + root + str(node.val) + '\n'
                   + diagram(node.left, bottom + '│ ', bottom + '└──', bottom + ' '))
        return diagram(self)

class AVLTree:
    def __init__(self):
        self.root = None

    def insert(self, val):
        self.root = self.__insert(self.root, val)

    def __insert(self, node, val):
        if not node:
            return AVLTreeNode(val)
        if val < node.val:
            node.left = self.__insert(node.left, val)
        else:
            node.right = self.__insert(node.right, val)
        balanced_node = self.balanced(node)
        balanced_node.height = max(balanced_node.get_left_height(),
                                   balanced_node.get_right_height()) + 1
        return balanced_node

    def left_rotate(self, node):
        pivot = node.right
        node.right = pivot.left
        pivot.left = node
        node.height = max(node.get_left_height(), node.get_right_height()) + 1
        pivot.height = max(pivot.get_left_height(), pivot.get_right_height()) + 1
        return pivot

    def right_rotate(self, node):
        pivot = node.left
        node.left = pivot.right
        pivot.right = node
        node.height = max(node.get_left_height(), node.get_right_height()) + 1
        pivot.height = max(pivot.get_left_height(), pivot.get_right_height()) + 1
        return pivot

    def right_left_rotate(self, node):
        if not node.right:
            return node
        node.right = self.right_rotate(node.right)
        return self.left_rotate(node)

    def left_right_rotate(self, node):
        if not node.left:
            return node
        node.left = self.left_rotate(node.left)
        return self.right_rotate(node)

    def balanced(self, node):
        if node.get_balanceFactor() == 2:
            if node.left and node.left.get_balanceFactor() == -1:
                return self.left_right_rotate(node)
            else:
                return self.right_rotate(node)
        if node.get_balanceFactor() == -2:
            if node.right and node.right.get_balanceFactor() == 1:
                return self.right_left_rotate(node)
            else:
                return self.left_rotate(node)
        return node

    def contain(self, val):
        current = self.root
        while current:
            if current.val == val:
                return True
            if val < current.val:
                current = current.left
            else:
                current = current.right
        return False

    def remove(self, val):
        self.root = self.__remove(self.root, val)

    def __remove(self, node, val):
        if not node:
            return None
        if val == node.val:
            if (not node.left) and (not node.right):
                return None
            if node.right:
                return node.right
            if node.left:
                return node.left
            node.val = node.right.get_min().val
            node.right = self.__remove(node.right, node.val)
        elif val < node.val:
            node.left = self.__remove(node.left, val)
        else:
            node.right = self.__remove(node.right, val)
        balanced_node = self.balanced(node)
        balanced_node.height = max(balanced_node.get_left_height(),
                                   balanced_node.get_right_height()) + 1
        return balanced_node

    def __repr__(self):
        return str(self.root)
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// swift
private func leftRotate(_ node: AVLNode<Element>) -> AVLNode<Element> {
        let pivot = node.right!
        node.right = pivot.left
        pivot.left = node
        node.height = max(node.leftHeight, node.rightHeight) + 1
        pivot.height = max(pivot.leftHeight, pivot.rightHeight) + 1
        return pivot
}

private func rightRotate(_ node: AVLNode<Element>) -> AVLNode<Element> {
    let pivot = node.left!
    node.left = pivot.right
    pivot.right = node
    node.height = max(node.leftHeight, node.rightHeight) + 1
    pivot.height = max(pivot.leftHeight, pivot.rightHeight) + 1
    return pivot
}

private func rightLeftRotate(_ node: AVLNode<Element>) -> AVLNode<Element> {
    guard let right = node.right else {
        return node
    }
    node.right = rightRotate(right)
    return leftRotate(node)
}

private func leftRightRotate(_ node: AVLNode<Element>) -> AVLNode<Element> {
    guard let left = node.left else {
        return node
    }
    node.left = leftRotate(left)
    return rightRotate(node)
}

private func balanced(_ node: AVLNode<Element>) -> AVLNode<Element> {
    switch node.balanceFactor {
    case 2:
        if let left = node.left, left.balanceFactor == -1 {
            return leftRightRotate(node)
        } else {
            return rightRotate(node)
        }
    case -2:
        if let right = node.right, right.balanceFactor == 1 {
            return rightLeftRotate(node)
        } else {
            return leftRotate(node)
        }
    default:
        return node
    }
}

Brute Force 蛮力法

selection sort 选择排序

{无论什么情况排序速度一样快}

  • 从第一个元素开始从它之后找最小的元素与之交换.
  • Time complexity: {Average: Θ(n^2), Worse: O(n^2), Best: Ω(n^2)}
  • Space complexity: {O(1)} 
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def selection_sort(list):
    for i in range(0, len(list) - 1):
        min_num = list[i]
        for j in range(i + 1, len(list)):
            min_num = min(min_num, list[j])
        list[list[i:].index(min_num) + i], list[i] = list[i], min_num
    return list

bubble sort 冒泡排序

{对于差不多排好序的速度很快，可以到达Ω(n)}

  • 比较相邻元素并将最大的元素向后沉直到最后，重复这个步骤
  • Time complexity: {Average: Θ(n^2), Worse: O(n^2), Best: Ω(n)}
  • Space complexity: {O(1)} 
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def bubble_sort(list):
    for i in range(len(list) - 1):
        for j in range(len(list) - 1 - i):
            if list[j] > list[j + 1]:
                list[j], list[j + 1] = list[j + 1], list[j]
    return list
def bubble_sort_upgrade(list):
    for i in range(len(list) - 1):
        already_sorted = True
        for j in range(len(list) - 1 - i):
            if list[j] > list[j + 1]:
                list[j], list[j + 1] = list[j + 1], list[j]
                already_sorted = False
        if already_sorted:
            break
    return list

sequential search 顺序查找 线性算法

  • Time complexity: {Average: Θ(n), Worse: O(n), Best: Ω(1)}
  • Space complexity: {O(1)} 
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def sequential_search(list, k):
    for i, ele in enumerate(list):
        if ele == k:
            return i
    return -1

dfs 深度优先查找

  • Time complexity: O(|V|+|E|) = O(b^{d})
  • Space complexity: O(|V|) 
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def dfs(graph, start):
    visited, stack, count = [], [start], 0
    while stack:
        vertex = stack.pop()
        count += 1
        visited.append(vertex)
        for v in graph[vertex]:
            if v not in visited + stack:
                stack.append(v)
    return visited, count

bfs 广度优先查找

  • Time complexity: O(|V|+|E|) = O(b^{d})
  • Space complexity: O(|V|) = O(b^{d}) 
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def bfs(graph, start):
    visited, queue, count = [], [start], 0
    while queue:
        vertex = queue.pop(0)
        count += 1
        visited.append(vertex)
        for v in graph[vertex]:
            if v not in visited + queue:
                queue.append(v)
    return visited, count

Decrease-And-Conquer 减治法

insertion sort 插入排序

{对于差不多排好序的速度很快，可以到达Ω(n)}

  • 从第二个元素开始向前找到正确的位置插入
  • Time complexity: {Average: Θ(n^2), Worse: O(n^2), Best: Ω(n)}
  • Space complexity: {O(1)} 
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def insetion_sort(list):
    for i in range(1, len(list)):
        insert_num = list[i]
        j = i - 1
        while j >= 0 and list[j] > insert_num:
            list[j + 1] = list[j]
            j -= 1
        list[j+ 1] = insert_num
    return list

shell sort 希尔排序

  • 通过一个gap来左插入排序(常用方法是从len/2gap开始每次缩小2倍)
  • Time complexity: {Average: Θ(n(logn)^2), Worse: O(n(logn)^2), Best: Ω(nlogn)}
  • Space complexity: {O(1)} 
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def shell_sort(list):
    gap = len(list) // 2
    while gap > 0:
        for i in range(gap, len(list)):
            temp = list[i]
            j = i
            while j >= gap and list[j - gap] > temp:
                list[j], list[j - gap] = list[j - gap], temp
                j -= gap
        gap = gap // 2
    return list

generating permutations

  • JohnsonTrotter
    • Time complexity: O(n!)

binary search 折半查找

  • Time complexity: {Average: Θ(logn), Worse: O(logn), Best: Ω(1)}
  • Space complexity: {O(1)} 
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def binary_search(sorted_list, target):
    low, high = 0, len(sorted_list) - 1
    while low <= high:
        mid = (low + high) // 2
        if sorted_list[mid] == target:
            return mid
        elif sorted_list[mid] > target:
            high = mid - 1
        else:
            low = mid + 1
    return -1

quick select 快速选择

  • 寻找第k个最小元素,通过划分来实现
  • Time complexity: {Average: Θ(n), Worse: O(n^2), Best: Ω(n)}
  • Space complexity: O(1) 
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def quick_select_m(list, start, end, k_th_min):
    s = partition(list, start, end)
    if s == k_th_min - 1:
        return list[s]
    elif s > k_th_min - 1:
        return quick_select_m(list, start, s - 1, k_th_min)
    else:
        return quick_select_m(list, s + 1, end, k_th_min)
def partition(list, start, end):
    pivot = list[start]
    i = start + 1
    j = end
    while True:
        while list[i] <= pivot and i <= j:
            i += 1
        while list[j] >= pivot and j >= i:
            j -= 1
        if i > j:
            break
        list[i], list[j] = list[j], list[i]
    list[start], list[j] = list[j], list[start]
    return j

Divide-And-Conquer 分治法

分解问题，求解子问题，合并自问题的解
T(n) = aT(n/b) + f(n) {a个需要求解的问题，问题被分成b个，f(n)的分解合并时间消耗}
T(n) = O(n^d) if a < b^d or O(n^dlogn) if a = b^d or O(n^log_b(a)) if a > b^d

merge sort 归并排序

{除了heapsort以外唯一BestAveWorst全是O(nlogn)排序算法}

  • 两种实现方法(自顶向下-递归， 自底向上-循环)
  • Time complexity: {Average: Θ(nlogn), Worse: O(nlogn), Best: Ω(nlogn)}
  • Space complexity: {O(n)} 
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def merge_sort(list):
    if len(list) > 1:
        list_b = list[:len(list) // 2]
        list_c = list[len(list) // 2:]
        return merge(merge_sort(list_b), merge_sort(list_c))
    else:
        return list
def merge(list_b, list_c):
    list_a = list_b + list_c    #init list_a
    i, j, k = 0, 0, 0
    while i < len(list_b) and j < len(list_c):
        if list_b[i] <= list_c[j]:
            list_a[k] = list_b[i]
            i += 1
        else:
            list_a[k] = list_c[j]
            j += 1
        k += 1
    if i == len(list_b):
        for index in range(j, len(list_c)):
            list_a[k] = list_c[index]
            k += 1
    if j == len(list_c):
        for index in range(i, len(list_b)):
            list_a[k] = list_b[index]
            k += 1
    return list_a

quick sort 快速排序

{pivot的选择对于算法效率至关重要}

  • 不断选择pivot来将元素划分到它的左右来实现
  • Time complexity: {Average: Θ(nlogn), Worse: O(n^2), Best: Ω(nlogn)}
  • Space complexity: {O(nlogn)}
  • pivot每次选第一个在已经排好序的数组上时间效率是O(n^2)
    • 优化方法
      • 使用随机pivot, 平均划分pivot, 快排好后使用插入排序 
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def quick_sort_m(list, start, end):
    if start < end:
        pivot = partition(list, start, end)
        quick_sort_m(list, start, pivot - 1)
        quick_sort_m(list, pivot + 1, end)
    return list
def partition(list, start, end):
    pivot = list[start]
    i = start + 1
    j = end
    while True:
        while list[i] <= pivot and i <= j:
            i += 1
        while list[j] >= pivot and j >= i:
            j -= 1
        if i > j:
            break
        list[i], list[j] = list[j], list[i]
    list[start], list[j] = list[j], list[start]
    return j

Transform-And-Conquer 变治法

  • 预排序解决问题
    • 检查数组中元素的唯一性
    • 算法数组的模式 （一个数组中最多的元素)

heap sort 堆排序

  • 先构建一个堆, 不断的删除最大键，
  • Time complexity: {Average: Θ(nlogn), Worse: O(nlogn), Best: Ω(nlogn)}
  • Space complexity: {O(1)} 
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import heapq
def heap_sort(list):
    heapq.heapify(list)
    return [heapq.heappop(list) for i in range(len(list))]

problem reduction 问题简化

{已有一种方法求其他问题}

  • lcm(m, n) gcd(m, n) = m n {lcm: 最小公倍数, gcd: 最大公约数}
  • 求一个函数的最小值， 可以求一个函数负函数的最大值的负数

hash table 散列表

  • 需要把键在hash table 中尽可能均匀分布
  • 平均插入，删除， 查找效率都是 O(1), 当最坏情况全部冲突到一个位置时候，退化到 O(n)
  • open hasing, also: separate chaining 分离链 开hash
  • closed hashing 闭hash
  • double hashing
  • rehasing

B-tree B树

Dynamic Programming (DP) 动态规划

  • 与其对交叠的子问题一次又一次地求解，还不如对每个较小的子问题只求解一次并把结果记录在表中。 (对具有交叠子问题的问题进行求解的技术)
    • 类似斐波那契数

coins_row problem (求互不相临的最大金币总金额)

  • Time complexity: O(n), Space complexity: O(n) 
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def coin_row(coins):
    coin1, coin2 = coins[0], coins[1]
    for i, coin in enumerate(coins[2:]):
        coin1, coin2 = coin2, max(coin1 + coin, coin2)
    return coin2

change_making problem

  • Time complexity: O(mn), Space complexity: O(n) 
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def change_making(coins, change):
    count = [0] * change
    count[0] = 0
    for i in range(1, change):
        tmp = float('inf')
        j = 0
        while j <= len(coins) and j < len(coins) and i >= coins[j]:
            tmp = min(count[i - coins[j]], tmp)
            j += 1
        count[i] = tmp + 1
    return count[-1]

knapsack problem 背包问题

  • Time complexity: O(nW), Space complexity: O(nW)
  • #TODO

memory function 记忆功能

TODO

Greedy Technique 贪婪技术

Prim

  • 构造最小生成树算法
  • 先随机选一个点，每次扩展新的点使得这个新的点到已有点的距离最短，直到添加完所有顶点 
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# TODO (heaven)

Kruskal

  • 构造最小生成树算法
  • 先按照权重将边进行排序，然后不断把边加入子图，如果加入此边会产生回路，则天国，直到完成 
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Dijkstra

  • 单起点最短路径问题 
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Bit Operation

  • get bit 
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def is_git_i_1(num, i):
    return num & (1 << i) != 0
  • set bit 
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def set_bit(num, i):
    return num | (1 << i)
  • clear bit 
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def clear_bit(num, i):
    mask = ~(1 << i)
    return num & mask
  • update bit 
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def update_bit(num, i, v):
    mask = ~(1 << i)
    return (num & mask) | (v << i)

How To Optimaize the Algorithm

  • Optimaize & Solve Technique

    • Look for BUD
      • Bottlenecks
      • Unnecessary work
      • Duplicated work
    • DIY (Do It Yourself)
    • Simplify and Generalize

    • Base Case and Build (always use for recursion)

      • like for generate permutations
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        1 Case "a" --> {"a"}
        2 Case "ab" --> {"ab", "ba"}
        3 Case "abc" --> {"cab", "acb", "abc", "cba", "bca", bac"}

    • Data structure brainstorm

      • hash
      • heap
      • tree

  • Consider the BCR (Best Conceivable Runtime)

    • The BCR is the best runtime you could conceive of a solution to a problem having.You can easily prove that there is no way you could beat the BCR.

  • Good coding looks like

    • correct
    • efficient
    • simple
    • readable
    • maintainable
      • use data structures generously
      • appropriate code resue
      • modular
      • flexible and robust
      • error checking

Power Table of 2

2^ = approximation approximate to
7 128
8 256
10 1024 one thousand 1K
16 65536 64K
20 1048576 one million 1MB
30 1073741824 one billion 1GB
32 4294967296 4GB
40 1099511627776 one trillion 1TB

Author: Min Gao

License: CC BY-NC-SA 4.0

如需转载或引用, 请标注出处。