三数中值快速排序（长度小于3的数组转插入排序）

 

 

void insertSort(int a[], int len)
{
    for (int i = 1; i < len; ++i)
    {
        int j;
        int temp = a[i];
        for (j = i - 1; j >= 0; --j)
        {
            if (a[j] <= temp)
                break;
            a[j + 1] = a[j];
        }
        a[j + 1] = temp;
    }
}

void swap(int *a, int *b)
{
    int temp = *a;
    *a = *b;
    *b = temp;
}

int median3(int a[], int left, int right)
{
    int center = (left + right) >> 1;
    if (a[left] > a[center])
        swap(&a[left], &a[center]);
    if (a[left] > a[right])
        swap(&a[left], &a[right]);
    if (a[center] > a[right])
        swap(&a[center], &a[right]);
    swap(&a[center], &a[right - 1]); // hide pivot
    return a[right - 1];
}

const int cutoff = 3;

void quickSort(int a[], int left, int right)
{
    int i, j;
    int pivot;
    if (left + cutoff <= right)
    {
        pivot = median3(a, left, right);
        i = left, j = right - 1;
        while (true)
        {
            while (a[++i] < pivot) {}
            while (a[--j] > pivot) {}
            if (i < j)
                swap(&a[i], &a[j]);
            else
                break;
        }
        swap(&a[i], &a[right - 1]); // restore pivot
        quickSort(a, left, i - 1);
        quickSort(a, i + 1, right);
    }
    else
        insertSort(a + left, right - left + 1);
}