快速排序算法（二）

1. 快速排序算法

　　在常用的快速排序算法中，每一步均将第一个数放到合适的位置。

　　下面介绍一种将任意数放到合适位置的快速排序算法。

int Partition(int data[], int length, int start, int end)
{
    if(data == NULL || length <= 0 || start <= 0 || end >=length)
        throw new std::exception("Invalid parameters");
    
    int index = RandomInRange(start, end);
    Swap(&data[index], &data[end]);
    
    int small = start - 1;
    for(index = start; index < end; index++)
    {
        if(data[index] < data[end])
        {
            small++;
            if(small != index)
                Swap(&data[index], &data[small]);
        }
    }
    
    small++;
    Swap(&data[small], &data[end]);
    
    return small;
}

int RandomInRange(start, end)
{
    int length = end - start + 1;
    int ret = rand() % length; //0 ~ length-1
}

void Swap(int* a, int* b)
{
    *b = *b + *a;
    *a = *b - *a;
    *b = *b - *a;
}

void QuickSort(int data[], int length, int start, int end)
{
    if(start == end)
        return;
    
    int index = Partition(data, length, start, end);
    if(index > start)
        QuickSort(data, length, start, index - 1);
    if(index < end)
        QuickSort(data, length, index + a, end);
}

2. 数组中第k大数字

int FindKthNumber(int data[], int length)
{
    int middle = length >> 1;
    int start = 0;
    int end = length - 1;
    int index = Partition(data, length, start, end);
    while(index != middle)
    {
        if(index > middle)
        {
            end = index - 1;
            index = Partition(data, length, start, end);
        }
        else
        {
            start = index + 1 ;
            index = Partition(data, length, start, end);
        }
    }
    
    return data[middle];
}