Quick sort and the Kth number

最新推荐文章于 2021-03-27 00:40:25 发布

原创最新推荐文章于 2021-03-27 00:40:25 发布 · 607 阅读

0 ·

CC 4.0 BY-SA版权

D&A 同时被 2 个专栏收录

21 篇文章

订阅专栏

Python

16 篇文章

订阅专栏

本文介绍了一种改进的快速排序算法——随机快速排序，并解释了如何使用此算法选择列表中的第k小元素。通过随机选择枢轴元素，提高了排序效率。此外，还详细阐述了选择特定元素的方法，提供了实现示例。

import random

# partition list L use the rightest element as pivot 
def partition(L, left, right):
    x = L[right]
    i = left - 1
    for j in range(left, right):
        if L[j] < x:
            i += 1
            L[i], L[j] = L[j], L[i]
    L[i + 1], L[right] = L[right], L[i + 1]
    return i + 1

# random partition
def random_partition(L, left, right):
    ''' get a pivot figure in list L
        the parameter left and right must be used in the random method
        because the random method is called recursively
    '''
    r = random.randrange(left,right)
    L[r], L[right] = L[right], L[r]

    return partition(L, left, right)

# random quick sort
def random_quicksort(L, left, right):
    if left < 0 or right > len(L):
        return
    if left < right:
        q = random_partition(L, left, right)
        #print q
        random_quicksort(L, left, q - 1)
        random_quicksort(L, q + 1, right)

# quick sort 
def quicksort(L, left, right):
    if left < 0 or right > len(L):
        return
    if left < right:
        q = partition(L, left, right)
        #print q
        quicksort(L, left, q - 1)
        quicksort(L, q + 1, right)

# select the kth number of the list L
def selectKth(L, left, right, k):
    assert k > 0 and k < len(L), "k must less than the length of the list"
    if left == right:
        return L[left]
    pivot = random_partition(L, left, right)
    #pivot = partition(L, left, right)
    d = pivot - left + 1
    #print d
    if k == d: # the pivot value is the answer
        return L[pivot]
    elif k < d:
        return selectKth(L, left, pivot - 1, k)
    else:
        return selectKth(L, pivot + 1, right, k - d)

def main():
    L = [1, 4, 3, 6, 5, 8, 7]
    print 'The original list is: ', L
    kth = selectKth(L, 0, 6, 1)
    print 'The kth number is ', kth
    
    random_quicksort(L, 0, 6)
    print 'The sorted list is: ', L
    
if __name__ == '__main__':
    main()