Python快速排序的实现方法

Python快速排序实现方法，具体代码如下：

 

# -*- coding: utf-8 -*-

 

from random import randint, shuffle

 

def _partition(seq, p, r):

"""数组划分，伪码如下：

PARTITION(A, p, r)

1 x ← A[r] // 作为划分主元

2 i ← p-1

3 for j ← p to r-1

4 do if A[j] <= x

5 then i ← i + 1 // 前划分区域的索引

6 exchange A[i] ↔A[j] // 小值交换到前面

7 exchange A[i+1] ↔A[r] // A[r]交换到前区域结尾

8 return i + 1

 

T(n) = θ(n)

"""

x = seq[r]

i = p - 1

for j in range(p, r):

if seq[j] <= x:

i += 1

seq[i], seq[j] = seq[j], seq[i]

i += 1

seq[i], seq[r] = seq[r], seq[i]

return i

 

def _quick_sort(seq, p, r):

"""递归调用，伪码如下：

QUICKSORT(A, p, r)

1 if p < r

2 then q ← PARTITION(A, p, r)

3 QUICKSORT(A, p, q-1)

4 QUICKSORT(A, q+1, r)

 

T(n) = θ(n^2)

"""

if p >= r:

return

q = _partition(seq, p, r)

_quick_sort(seq, p, q - 1)

_quick_sort(seq, q + 1, r)

 

def quick_sort(seq):

"""快速排序

Args:

seq (Sequence): 一个序列对象。

"""

_quick_sort(seq, 0, len(seq) - 1)

 

 

def _rand_partition(seq, p, r):

"""随机取样交换后再划分，伪码如下：

RANDOMIZED-PARTITION(A, p, r)

1 i ← RANDOM(p, r) // 从A[p..r]中随机选出一个

2 exchange A[r] ↔ A[i] // A[r]与其交换

3 return PARTITION(A, p, r)

 

T(n) = O(n)

"""

i = randint(p, r)

seq[r], seq[i] = seq[i], seq[r]

return _partition(seq, p, r)

 

def _rand_qsort(seq, p, r):

"""随机取样划分方式的递归调用，伪码如下：

RANDOMIZED-QUICKSORT(A, p, r)

1 if p < r

2 then q ← RANDOMIZED-PARTITION(A, p, r)

3 RANDOMIZED-QUICKSORT(A, p, q-1)

4 RANDOMIZED-QUICKSORT(A, q+1, r)

 

T(n) = O(n^2)

"""

if p >= r:

return

q = _rand_partition(seq, p, r)

_rand_qsort(seq, p, q - 1)

_rand_qsort(seq, q + 1, r)

 

def rand_qsort(seq):

"""快速排序（随机化版本）"""

_rand_qsort(seq, 0, len(seq) - 1)

 

 

def qsort(L):

"""快速排序（简易版本）

更多: Python Cookbook 2nd Edition 第5.11章节

"""

if not L: return []

return qsort([x for x in L[1:] if x < L[0]]) + L[0:1] + \

qsort([x for x in L[1:] if x >= L[0]])

 

if __name__ == '__main__':

import timeit

 

items = range(10000)

shuffle(items)

 

def test_sorted():

print(items)

sorted_items = sorted(items)

print(sorted_items)

 

def test_quick_sort():

print(items)

quick_sort(items)

print(items)

 

test_methods = [test_sorted, test_quick_sort] # test_rand_qsort, test_qsort

for test in test_methods:

name = test.__name__ # test.func_name

t = timeit.Timer(name + '()', 'from __main__ import ' + name)

print(name + ' takes time : %f' % t.timeit(1))转载请注明来源管理学家 http://www.400qikan.com/qk/1297/