跟着两位大佬的步伐继续学算法，这次是归并排序，还是直接上代码。。。

• http://blog.csdn.net/jofranks/article/details/7802447
• http://blog.csdn.net/morewindows/article/details/6678165/

代码

#include <iostream>

using namespace std;

void mergeArray(int* a, int first, int mid, int last, int* res) //将两个有序数列合并
{
    int i = 0;
    int left = first;
    int right = mid + 1;

    while(left <= mid && right <= last)
    {
        if(a[left] < a[right])
            res[i++] = a[left++];
        else
            res[i++] = a[right++];
    }

    while(left <= mid)
        res[i++] = a[left++];

    while(right <= last)
        res[i++] = a[right++];

    for(int j = 0; j < i; j++)  //将最终结果放到原数组里，删除临时数组res，减少new操作，节省时间
        a[first+j] = res[j];
}

void mergeSort(int* a, int first, int last, int* res)
{
    if(first < last)
    {
        int mid = (first + last) / 2;
        mergeSort(a, first, mid, res);    //数列左边递归分解，使其有序
        mergeSort(a, mid+1, last, res);   //右边
        mergeArray(a, first, mid, last, res);   //合并左右序列
//        for(int i = 0; i < 10; i++)
//            cout << a[i] << ' ';
//        cout << endl;
    }
}

int main()
{
    int num;
    cin >> num;

    int* res = new int [num];
    int* a = new int [num];
    for(int i = 0; i < num; i++)
        cin >> a[i];

    mergeSort(a, 0, num-1, res);

    for(int i = 0; i < num; i++)
        cout << a[i] << ' ';
    cout << endl;

    delete [] a;
    delete [] res;
    return 0;
}