快排（qucikSort）

#include <iostream>

using namespace std;

void  QSort(int a[], int low, int high)
{
    if(low>=high)  //关于这个终止条件， 在下边处理后，low可能会大于或等于high
    {
        return;
    }
    int first = low;
    int last = high;
    int key = a[first];   //use the first elements to be the key
    while(first<last)
    {

        while(first<last&&a[last]>=key)
        {
            last--;
        }
        a[first] = a[last];

        while(first<last&&a[first]<=key)
        {
            first++;
        }

        a[last] = a[first];  //we always put the value of first into the last, so the first is NULL at last

        a[first] = key;     //insert the centre elements
        QSort(a, low, first-1);
        QSort(a, first+1, high);

    }

}

int main()
{
    int a[] = {57, 68, 59, 52, 72, 28, 96, 33, 24};
    int n = sizeof(a)/sizeof(a[0]);
    QSort(a, 0, n-1); //third  parameter minus one, otherwise array will overstep the boundary
    for(int i=0; i<n; i++)
        cout << a[i] << " "; 
    return 0;
}