快速排序

其实研究算法的时候，最好的做法就是拿出纸和笔，然后自己一行一行代码的走下去。

这样学习的速度我觉得是最快的。

快排的基本思想是：通过一次排序将整个序列分成两部分，其中一部分的记录均比关

键值小，另一部均比关键值大。然后在分别对两个小部分进行同样的思想处理。

 

代码：

#include <iostream>

using namespace std;

void Quicksort(int *iArr, int low, int high);
int Partition(int *iArr, int low, int high);
//int Partition2(int *iArr, int low, int high);
void Swap(int &i, int &j)
{
    int iTemp = i;
    i = j;
    j = iTemp;
}

void Display(int iArr[], int iLen)
{
    std::cout << "iArr = ";
    for(int i = 0; i < iLen; i++)
    {
         std::cout << "\t" << iArr[i];
    }   
    std::cout << std::endl;
}

int main(int argc, char **argv)
{
    int low = 0;
    int high = 7;
    int iArr[8] = {2, 8, 7, 1, 3, 5, 6, 4};

    Display(iArr, 8);
    Quicksort(iArr, low, high);
    Display(iArr, 8);
    return 0;
}

void Quicksort(int *iArr, int low, int high)
{
    if(low < high)
    {
        int iPivotLoc = Partition(iArr, low, high);
        Quicksort(iArr, low, iPivotLoc - 1);
        Quicksort(iArr, iPivotLoc + 1, high);
    }
}

int Partition(int *iArr, int low, int high)
{
    while(low < high)
    {
        int iPivot = iArr[low];

        while(low < high && iArr[high] >= iPivot)
        {
            high--;
        }

        if(low != high)
        {
            Swap(iArr[low], iArr[high]);
        }

        while(low < high && iArr[low] <= iPivot)
        {
            low++;
        }

        if(low != high)
        {
            Swap(iArr[low], iArr[high]);
        }
    }

    return low;
}

整个算法的执行过程如下：(建议按照这个数据，然后用纸和笔手动的走一遍代码。)

iArr = 2 8 7 1 3 5 6 4
low = 0 high = 7 iPivot = 2
low = 0 high = 6 iPivot = 2
low = 0 high = 5 iPivot = 2
low = 0 high = 4 iPivot = 2
low = 0 high = 3 iPivot = 2
low = 0 high = 3 iPivot = 2 // 53行为false了
iArr = 1 8 7 2 3 5 6 4
low = 1 high = 3 iPivot = 2
low = 1 high = 3 iPivot = 2 // 63行为false了
iArr = 1 2 7 8 3 5 6 4
low = 1 high = 3 iPivot = 2
low = 1 high = 2 iPivot = 2
low = 1 high = 1 iPivot = 2
low = 1 high = 1 iPivot = 2 // 53行为false了
iArr = 1 2 7 8 3 5 6 4
low = 1 high = 1 iPivot = 2 // 63行为false了
iArr = 1 2 7 8 3 5 6 4
***************************************************** // 整个Partition函数执行完了

iArr = 1 2 7 8 3 5 6 4
low = 2 high = 7 iPivot = 7
low = 2 high = 7 iPivot = 7
iArr = 1 2 4 8 3 5 6 7
low = 3 high = 7 iPivot = 7
low = 3 high = 7 iPivot = 7
iArr = 1 2 4 7 3 5 6 8
low = 3 high = 7 iPivot = 7
low = 3 high = 6 iPivot = 7
low = 3 high = 6 iPivot = 7
iArr = 1 2 4 6 3 5 7 8
low = 4 high = 6 iPivot = 7
low = 5 high = 6 iPivot = 7
low = 6 high = 6 iPivot = 7
low = 6 high = 6 iPivot = 7
iArr = 1 2 4 6 3 5 7 8
*****************************************************

iArr = 1 2 4 6 3 5 7 8
low = 2 high = 5 iPivot = 4
low = 2 high = 4 iPivot = 4
low = 2 high = 4 iPivot = 4
iArr = 1 2 3 6 4 5 7 8
low = 3 high = 4 iPivot = 4
low = 3 high = 4 iPivot = 4
iArr = 1 2 3 4 6 5 7 8
low = 3 high = 4 iPivot = 4
low = 3 high = 3 iPivot = 4
low = 3 high = 3 iPivot = 4
iArr = 1 2 3 4 6 5 7 8
low = 3 high = 3 iPivot = 4
iArr = 1 2 3 4 6 5 7 8
*****************************************************

iArr = 1 2 3 4 6 5 7 8
low = 4 high = 5 iPivot = 6
low = 4 high = 5 iPivot = 6
iArr = 1 2 3 4 5 6 7 8
low = 5 high = 5 iPivot = 6
low = 5 high = 5 iPivot = 6
iArr = 1 2 3 4 5 6 7 8
*****************************************************

最终结果：
iArr = 1 2 3 4 5 6 7 8

 

好像还以一种更高效一点的写法，后续更新。