/*3元取中的快速排序

*

* 伪代码：

* PARTITION(A, p, r)

* 1 x ← A[r] //以最后一个元素，A[r]为主元

* 2 i ← p - 1

* 3 for j ← p to r - 1 //注，j从p指向的是r-1，不是r。

* 4 do if A[j] ≤ x

* 5 then i ← i + 1

* 6 exchange A[i] <-> A[j]

* 7 exchange A[i + 1] <-> A[r] //最后，交换主元

* 8 return i + 1

*

* QUICKSORT(A, p, r)

* 1 if p < r

* 2 then q ← PARTITION(A, p, r) //关键

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

* 4 QUICKSORT(A, q + 1, r)

*

*/

#include <stdio.h>

#define N 20

#define MAX1(a,b) (a)>(b)?(a):(b)

#define MAX(a,b,c) (a)>(MAX1(b,c))?(a):(MAX1(b,c))

/*打印数组*/

void print_arr(int *a, int n)

{

int i;

for(i = 0; i < n; i++)

printf("%d ",a[i]);

putchar('\n');

}

/*交换两个数的值*/

void swap(int *a, int *b)

{

int temp;

temp = *a;

*a = *b;

*b = temp;

}

int Getmid(int *A,int left, int right)

{

int a = -1, b = -1, c = -1;

int max;

//取随机的3元

/*

while(a < left)

a = rand() % right;

while(b < left)

b = rand() % right;

while(c < left)

c = rand() % right;

*/

//取第一 中间 及最后一个元素为3元

a = left; b = (left + right)/2; c = right;

//return MAX(A[a],A[b],A[c]);

if(A[a] > A[b])

max = a;

else max = b;

if(A[c] > A[max])

max = c;

return max;

}

/*快排的核心，划分数组

* 选定一个主元key，将数组划分为

* [ (>key) key (<key)]

*/

int partition(int *a, int left, int right)

{

int i,j,m;

int key;

m = Getmid(a,left,right);

swap(a+m,a+right);

key = a[right]; //选末尾元素为主元key

i = left - 1; // [a0,a1,a2,a3,a4,a5,a6,a7,a8,a9]

for(j = left; j <= right-1; j++) // i j key

if(a[j] >= key)

{

i++;

swap(a+i,a+j);

}

swap(a+i+1,a+right);

return i+1;

}

/*快排主程序*/

void q_sort(int *a, int left, int right)

{

int point;

if(left < right) //base case

{

point = partition(a,left,right);

q_sort(a,left,point - 1);

q_sort(a,point + 1,right);

}

}

void quicksort(int *a, int n)

{

q_sort(a,0,n-1);

}

int main(int argc, char **argv)

{

int array[N];

int i;

srand((unsigned)time(NULL));

for(i = 0; i < N; i++)

array[i] = rand() % 100;

print_arr(array,N);

quicksort(array,N);

print_arr(array,N);

return 0;

}