快速选择算法的一个简单实现

#include<stdio.h>//交换两个元素void swap(int *a, int *b){int temp;temp = *a;*a = *b;*b = temp;}//三数取中void median3(int a[], int left, int right){int middle = (left+right)/2;if(a[left]>a[middle])swap(&a[left], &a[middle]);if(a[left]>a[right])swap(&a[left], &a[right]);if(a[middle]>a[right])swap(&a[middle],&a[right]);swap(&a[left], &a[middle]);}

//划分（快速排序思想）int partition(int a[], int left, int right){if(left<=right){int i = left;int j = right;int temp = a[left];while(i<j){while(i<j&&a[j]>temp){j--;}swap(&a[i], &a[j]);while(i<j && a[i]<temp)i++;swap(&a[i], &a[j]);}a[i] = temp;return i;}}

//快速选择算法

int quickSelecting(int a[], int left, int right, int k){if(left<=right){median3(a, left, right);int position = partition(a,left,right);if(k-1 == position)return a[position];else if(k-1 < position)quickSelecting(a,left,position-1,k);else if(k-1 > position)quickSelecting(a,position+1,right,k);}}int main(){int a[10] = {8, 1, 4 ,9, 6, 3, 5, 2, 7, 0};int k;printf("input the value of k:\n");scanf("%d",&k);int result = quickSelecting(a,0,9,k);printf("the result is %d\n",result);return 1;}