算法导论 第七章 快速排序（python）

用的最多的排序

平均性能：O(nlogn){随机化nlogn}

原地址排序

稳定性：不稳定

思想：分治 （切分左右）

学习方式：自己在纸上走一遍

def PARTITION(A,p,r):
    x = A[r] # 锚点 主元{大于它放一边，小于的放另一边}
    i = p - 1
    for j in range(p,r):
        if A[j] <= x:
            i += 1
            A[i],A[j] = A[j],A[i]
    A[i+1],A[r] = A[r],A[i+1]
    return i + 1

def QUICKSORT(A,p,r):
    if p < r: #分治
        q = PARTITION(A,p,r)
        QUICKSORT(A,p,q-1)
        QUICKSORT(A,q+1,r)

if __name__ == "__main__":
    A = [2,8,7,1,3,5,6,4]
    QUICKSORT(A,0,len(A)-1)
    print(A)
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=============== RESTART: F:/python/algorithms/7_1_quicksort.py ===============
[1, 2, 3, 4, 5, 6, 7, 8]

win7+python3.5.1

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随机快速排序

import random

def PARTITION(A,p,r):
    x = A[r] # 锚点 主元{大于它放一边，小于的放另一边}
    i = p - 1
    for j in range(p,r):
        if A[j] <= x:
            i += 1
            A[i],A[j] = A[j],A[i]
    A[i+1],A[r] = A[r],A[i+1]
    return i + 1

def RANDOMIZED_PARTITION(A,p,r):
    i = random.randint(p,r) #随机的变化
    A[i],A[r] = A[r],A[i]
    return PARTITION(A,p,r)

def RANDOMIZED_QUICKSORT(A,p,r):
    if p < r: #分治
        q = RANDOMIZED_PARTITION(A,p,r)
        RANDOMIZED_QUICKSORT(A,p,q-1)
        RANDOMIZED_QUICKSORT(A,q+1,r)

if __name__ == "__main__":
    A = [2,8,7,1,3,5,6,4]
    RANDOMIZED_QUICKSORT(A,0,len(A)-1)
    print(A)
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=============== RESTART: F:/python/algorithms/7_1_quicksort.py ===============
[1, 2, 3, 4, 5, 6, 7, 8]

win7+python3.5.1

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