归并排序

归并排序（Merge sort）是建立在归并操作上的一种有效的排序算法。该算法是采用分治法（Divide and Conquer）的一个非常典型的应用。

归并操作(merge)，也叫归并算法，指的是将两个已经排序的序列合并成一个序列的操作。归并排序算法依赖归并操作。

// 参考白话经典算法之归并排序的思想
#include <stdio.h>
#include <malloc.h>

void SwapValue(int *OperatorA, int *OperatorB)
{
    if ((NULL == OperatorA) || (NULL == OperatorB))
    {
        printf ("Invalid Parameter(s)!\n");
        return;
    }

    if ((*OperatorA) != (*OperatorB))
    {
        *OperatorA ^= *OperatorB;
        *OperatorB ^= *OperatorA;
        *OperatorA ^= *OperatorB;
    }
}

void CombineArray(int *a, int fStart, int fEnd, int sStart, int sEnd)
{
    if ((NULL == a) || (fStart < 0) || (fStart > fEnd) || (sStart < 0) || (sStart > sEnd))
    {
        printf ("Invalid Parameter(s)!\n");
        return;
    }

    int *pTemp = NULL;

    if (NULL == (pTemp = (int *)malloc((fEnd - fStart + 1 + sEnd - sStart + 1) * sizeof(int))))
    {
        printf ("Fail to malloc space to pTemp!\n");
        return;
    }

    if (fStart > sStart)
    {
        SwapValue (&fStart, &sStart);
        SwapValue (&fEnd, &sEnd);
    }

    int i = fStart;
    int j = sStart;
    int k = 0;

    while ((i <= fEnd) && (j <= sEnd))
    {
        if (a[i] < a[j])
        {
            pTemp[k++] = a[i++];
        }
        else
        {
            pTemp[k++] = a[j++];
        }
    }

    while (i <= fEnd)
    {
        pTemp[k++] = a[i++];
    }

    while (j <= sEnd)
    {
        pTemp[k++] = a[j++];
    }

    k = 0;
    for (i = fStart; i <= fEnd; ++i)
    {
        a[i] = pTemp[k++];
    }

    for (i = sStart; i <= sEnd; ++i)
    {
        a[i] = pTemp[k++];
    }
}

void MergeSort(int *a, int nLow, int nHigh)
{
    if ((NULL == a) || (nLow < 0) || (nLow > nHigh))
    {
        printf ("Invalid Parameter(s)!\n");
        return;
    }

    if (nLow < nHigh)
    {
        int nMid = (nLow + nHigh) >> 1;

        // 左边有序
        MergeSort (a, nLow, nMid);

        // 右边有序
        MergeSort (a, nMid + 1, nHigh);

        // 合并有序
        CombineArray (a, nLow, nMid, nMid + 1, nHigh);
    }
}

int main(void)
{
    int a1[9] = {9, 8, 7, 6, 5, 4, 3, 2, 1};

    MergeSort (a1, 0, 8);

    for (int i = 0; i < 9; ++i)
    {
        printf ("%d ", a1[i]);
    }

    printf ("\n");

    return 0;
}