在C语言的库函数中就有快速排序的库函数，即为qsort

在C语言的库函数中就有快速排序的库函数，即为qsort, 其用法如下：

功 能： 快速排序
头文件：stdlib.h
用 法： void qsort(void *base,int nelem,int width,int (*fcmp)(const void *,const void *));
参数：
1 待排序数组首元素的地址
2 数组中待排序元素数量
3 各元素的占用空间大小
4 指向函数的指针，用于确定排序的顺序

示例程序如下：

[cpp]
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#include <stdio.h>
#include <stdlib.h>

int compInc(const void *a, const void *b)
{
return *(int *)a - *(int *)b;
}

int compDec(const void *a, const void *b)
{
return *(int *)b - *(int *)a;
}

int main()
{
int a[5] = {11,2,13,4,7};
int b[5] = {11,2,13,4,7};
int len = 5;
int i;

printf("递增排序结果:\n");
qsort(a, len, sizeof(a[0]), compInc);
for(i = 0; i < len; i ++)
{
printf("%d ", a[i]);
}

printf("\n\n");

printf("递减排序结果:\n");
qsort(b, len, sizeof(b[0]), compDec);

for(i = 0; i < len; i ++)
{

printf("%d ", b[i]);
}

printf("\n");

return 0;
}

#include <stdio.h>
#include <stdlib.h>

int compInc(const void *a, const void *b)
{
return *(int *)a - *(int *)b;
}

int compDec(const void *a, const void *b)
{
return *(int *)b - *(int *)a;
}

int main()
{
int a[5] = {11,2,13,4,7};
int b[5] = {11,2,13,4,7};
int len = 5;
int i;

printf("递增排序结果:\n");
qsort(a, len, sizeof(a[0]), compInc);
for(i = 0; i < len; i ++)
{
printf("%d ", a[i]);
}

printf("\n\n");

printf("递减排序结果:\n");
qsort(b, len, sizeof(b[0]), compDec);

for(i = 0; i < len; i ++)
{

printf("%d ", b[i]);
}

printf("\n");

return 0;
}

结果为：
递增排序结果:

2 4 7 11 13

递减排序结果:

13 11 7 4 2

下面，继续看qsort的一些用法：

[cpp]
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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define M 12
#define N 20

int compareInc(const void *a, const void *b)
{
return strlen((char *)a) - strlen((char*)b);
}

int compareDec(const void *a, const void *b)
{
return strlen((char *)b) - strlen((char*)a);
}

int main(void)
{
int i;
char s[M]
=
{
"January",
"February",
"March",
"April",
"May",
"June",
"July",
"August",
"September",
"October",
"November",
"December"
};

qsort(s, M, sizeof(char) * N, compareInc);
for(i = 0;i < M; i++)
printf("%s\n", s[i]);

printf("\n");

qsort(s, M, sizeof(char) * N, compareDec);
for(i = 0;i < M; i++)
printf("%s\n", s[i]);

return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define M 12
#define N 20

int compareInc(const void *a, const void *b)
{
return strlen((char *)a) - strlen((char*)b);
}

int compareDec(const void *a, const void *b)
{
return strlen((char *)b) - strlen((char*)a);
}

int main(void)
{
int i;
char s[M]
=
{
"January",
"February",
"March",
"April",
"May",
"June",
"July",
"August",
"September",
"October",
"November",
"December"
};

qsort(s, M, sizeof(char) * N, compareInc);
for(i = 0;i < M; i++)
printf("%s\n", s[i]);

printf("\n");

qsort(s, M, sizeof(char) * N, compareDec);
for(i = 0;i < M; i++)
printf("%s\n", s[i]);

return 0;
}

结果为：
May

July

June

March

April

August

October

January

December

November

February

September

September

February

November

December

October

January

August

April

March

June

July

May

[cpp]
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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define M 12
#define N 20

int compare1(const void *a, const void *b)
{
return *(char *)a - *(char*)b;
}

int compare2(const void *a, const void *b)
{
return *(char *)b - *(char*)a;
}

int main(void)
{
int i;
char s[M]
=
{
"January",
"February",
"March",
"April",
"May",
"June",
"July",
"August",
"September",
"October",
"November",
"December"
};

qsort(s, M, sizeof(char) * N, compare1);
for(i = 0;i < M; i++)
printf("%s\n", s[i]);

printf("\n");

qsort(s, M, sizeof(char) * N, compare2);
for(i = 0;i < M; i++)
printf("%s\n", s[i]);

return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define M 12
#define N 20

int compare1(const void *a, const void *b)
{
return *(char *)a - *(char*)b;
}

int compare2(const void *a, const void *b)
{
return *(char *)b - *(char*)a;
}

int main(void)
{
int i;
char s[M]
=
{
"January",
"February",
"March",
"April",
"May",
"June",
"July",
"August",
"September",
"October",
"November",
"December"
};

qsort(s, M, sizeof(char) * N, compare1);
for(i = 0;i < M; i++)
printf("%s\n", s[i]);

printf("\n");

qsort(s, M, sizeof(char) * N, compare2);
for(i = 0;i < M; i++)
printf("%s\n", s[i]);

return 0;
}

结果为：
April

August

December

February

July

June

January

March

May

November

October

September

September

October

November

May

March

June

July

January

February

December

August

April

好，欣赏最后一个qsort程序：

[cpp]
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#include <stdio.h>
#include <stdlib.h>
#define N 6

typedef struct
{
char name[15];
int score;

}Student;

int compare1(const void *a,const void *b)
{
return ((Student*)a)->score - ((Student*)b)->score;
}

int compare2(const void *a,const void *b)
{
return *(((Student*)a)->name) - *(((Student*)b)->name);
}

void print(Student s)
{
printf("%-15s : %d\n", s.name, s.score);
}

int main()
{
Student s
=
{
"Zhang San", 94,
"Li Si", 80,
"You", 94,
"I", 100,
"He", 72,
"She", 60
};

int i;
qsort(s, N, sizeof(Student), compare1);
for(i = 0; i < N; i++)
{
print(s[i]);
}

printf("\n");

qsort(s, N, sizeof(Student), compare2);
for(i = 0; i < N; i++)
{
print(s[i]);
}

return 0;
}

#include <stdio.h>
#include <stdlib.h>
#define N 6

typedef struct
{
char name[15];
int  score;

}Student;

int compare1(const void *a,const void *b)
{
return ((Student*)a)->score - ((Student*)b)->score;
}

int compare2(const void *a,const void *b)
{
return *(((Student*)a)->name) - *(((Student*)b)->name);
}

void print(Student s)
{
printf("%-15s : %d\n", s.name, s.score);
}

int main()
{
Student s
=
{
"Zhang San", 94,
"Li Si",     80,
"You",       94,
"I",        100,
"He",        72,
"She",       60
};

int i;
qsort(s, N, sizeof(Student), compare1);
for(i = 0; i < N; i++)
{
print(s[i]);
}

printf("\n");

qsort(s, N, sizeof(Student), compare2);
for(i = 0; i < N; i++)
{
print(s[i]);
}

return 0;
}

结果为：
She : 60

He : 72

Li Si : 80

You : 94

Zhang San : 94

I : 100

He : 72

I : 100

Li Si : 80

She : 60

You : 94

Zhang San : 94