算法学习2：快速排序

快速排序算法的基本思想就是选择一个基准数，然后把小于基准数的数都扔到基准数左边，大于基准数的数都放到基准数右边，然后对基准数左右两边的两个子数组再重复这一过程（递归调用）。

示例：

#include<stdio.h>
int a[101], n;  // define 2 global variables

void quicksort(int left, int right)
{
    int i, j, t, temp;
    if (left > right)
    {
        return;
    }

    temp = a[left];  // base value
    i = left;
    j = right;
    while (i != j)
    {
        // from right to left
        while (a[j] >= temp && i < j)
        {
            j--;
        }
        // from left to right
        while (a[i] <= temp && i < j)
        {
            i++;
        }

        if (i < j) // if the 2 sentinals didn't meet
        {
            // swap the 2 numbers
            t = a[i];
            a[i] = a[j];
            a[j] = t;
        }
    }

    // set back the base value
    a[left] = a[i];
    a[i] = temp;

    // recursive call
    quicksort(left, i - 1);  // quick sort the left sub sequence
    quicksort(i + 1, right); // quick sort the right sub sequence
}

int main()
{
    int i, j, t;
    scanf_s("%d",&n);
    for (i = 0; i < n; i++)
    {
        scanf_s("%d", &a[i]);
    }

    quicksort(0, n-1);

    for (i = 0; i < n; i++)
    {
        printf("%d ", a[i]);
    }

    getchar(); getchar();
    return 0;
}