经典排序算法学习笔记二——快速排序

快速排序

1、算法思想

2、伪代码

function quicksort(q)//from 维基百科
var list less, pivotList, greater
if length(q) ≤ 1 {
return q
} else {
select a pivot value pivot from q
for each x in q except the pivot element
if x < pivot then add x to less
if x ≥ pivot then add x to greater
add pivot to pivotList
return concatenate(quicksort(less), pivotList, quicksort(greater))
}

QUICK_SORT(A,p,r)//from 算法导论
if(p<r)
thenq<——PARTITION(A,p,r)
QUICK_SORT(A,p,q-1)
QUICK_SORT(A,q+1,r)

//核心函数，对数组A[p,r]进行就地重排，将小于A[r]的数移到数组前半部分，将大于A[r]的数移到数组后半部分。
//
PARTITION(A,p,r)
pivot<——A[r]
i<——p-1
forj<——ptor-1
doifA[j]<pivot
i<——i+1
exchangeA[i]<——>A[j]
exchangeA[i+1]<——>A[r]
return i+1

3、C实现

迭代法

typedef struct _Range {
int start, end;
} Range;
Range new_Range(int s, int e) {
Range r;
r.start = s;
r.end = e;
return r;
}
void swap(int *x, int *y) {
int t = *x;
*x = *y;
*y = t;
}
void quick_sort(int arr[], const int len) {
if (len <= 0)
return; //避免len等於負值時宣告堆疊陣列當機
//r[]模擬堆疊,p為數量,r[p++]為push,r[--p]為pop且取得元素
Range r[len];
int p = 0;
r[p++] = new_Range(0, len - 1);
while (p) {
Range range = r[--p];
if (range.start >= range.end)
continue;
int mid = arr[range.end];
int left = range.start, right = range.end - 1;
while (left < right) {
while (arr[left] < mid && left < right)
left++;
while (arr[right] >= mid && left < right)
right--;
swap(&arr[left], &arr[right]);
}
if (arr[left] >= arr[range.end])
swap(&arr[left], &arr[range.end]);
else
left++;
r[p++] = new_Range(range.start, left - 1);
r[p++] = new_Range(left + 1, range.end);
}
}

递归法

#include <stdio.h>
#include <stdlib.h>
void swap(int *x, int *y) {
int t = *x;
*x = *y;
*y = t;
}
void quick_sort_recursive(int arr[], int start, int end) {
if (start >= end)
return;//這是為了防止宣告堆疊陣列時當機
int mid = arr[end];
int left = start, right = end - 1;
while (left < right) {
while (arr[left] < mid && left < right)
left++;
while (arr[right] >= mid && left < right)
right--;
swap(&arr[left], &arr[right]);
}
if (arr[left] >= arr[end])
swap(&arr[left], &arr[end]);
else
left++;
quick_sort_recursive(arr, start, left - 1);
quick_sort_recursive(arr, left + 1, end);
}
void quick_sort(int arr[], int len) {
quick_sort_recursive(arr, 0, len - 1);
}
int main() {
int arr[] = { 22, 34, 3, 32, 82, 55, 89, 50, 37, 5, 64, 35, 9, 70 };
int len = (int) sizeof(arr) / sizeof(*arr);
int i;
printf("before quick_sort:\n");
for (i = 0; i < len; i++)
printf("%d ", arr[i]);
quick_sort(arr, len);
printf("\nquick_sorted:\n");
for (i = 0; i < len; i++)
printf("%d ", arr[i]);
system("pause");
return 0;
}

4、改进