快速排序，平均時間復雜度O(nlogn)，

#include<stdio.h>
#include<iostream>
using namespace std;
int partition(int a[],int left,int right){
    int x=a[left];
    while(left<right){
        while(left<right&&a[right]>=x)    --right;
        a[left]=a[right];
        while(left<right&&a[left]<=x)    ++left;
        a[right]
 
=a[left];
    }
    a[left]=x;
    return left;
}
void quick_sort(int a[],int left,int right){
    if(left==right)    return ;
    int index=partition(a,left,right);
    if(index>left)
        quick_sort(a,left,index-1);
    if(index<right)
        quick_sort(a,index+1,right);
}
int
 
 main(){
    int a[]={1,2,4,3,2};
    quick_sort(a,0,4);
    for(int i=0;i<=4;i++)
        printf("%d ",a[i]);
    return 0;
}

排序算法實現