归并排序

参考：白话经典算法系列之五 归并排序的实现

#include "stdafx.h"

//t[]在外部申请，以此避免在函数体内部反复申请数组的开销。
void merge_array(int a[], int left, int middle, int right, int t[]) 
{
    int l = left, r = middle + 1, k = 0;
    while(l <= middle && r <= right)
    {
        if(a[l] < a[r])
             t[k++] = a[l++];
        else
            t[k++] = a[r++];
    }

    while(l <= middle)
        t[k++] = a[l++];

    while(r <= right)
        t[k++] = a[r++];

    for(int i = 0; i < k; ++i)
        a[left + i] = t[i];       //mark!第一次忘记写left了。
}

void merge_sort(int a[], int left, int right, int t[])
{
    if(left >= right) return;
    int middle = (left + right)/2;
    merge_sort(a, left, middle, t);
    merge_sort(a, middle + 1, right, t);
    merge_array(a, left, middle, right, t);    
}

int _tmain(int argc, _TCHAR* argv[])
{
    int array[] = {7,10,19,25,12,17,21,30,48};
    
    int * t = new int[sizeof(array)/sizeof(int)];    
    merge_sort(array, 0, sizeof(array)/sizeof(int) - 1, t);
    delete []t;

    getchar();
    return 0;
}