歸併排序詳解（戳我）。

以下是搬了別人的。

#include<stdio.h>
#include<stdlib.h>

long long sum = 0;

int a[100005];
int temp[100005];


void Merge(int s1, int e1, int s2, int e2) 
{
    int p = 0;
    int p1 = s1;
    int p2 = s2;
    while(p1 <= e1 && p2 <= e2)
    {
        if(a[p1] <= a[p2])
        {
            temp[p++] = a[p1 ++];
        }
        else
        {
            temp[p ++] = a[p2 ++];
            sum += (e1 - p1 + 1);
        }
    }
    while(p1 <= e1)
    {
        temp[p++] = a[p1++];
    }
    while(p2 <= e2)
    {
        temp[p++] = a[p2++];
    }
    for(int i = s1; i <= e2; i ++)
    {
        a[i] = temp[i - s1];
    }
}

void Merge_sort(int s, int e)
{
    int m;
    if(s < e)
    {
         m = ( s + e ) / 2;
        Merge_sort(s, m);
        Merge_sort(m + 1, e);
        Merge(s, m , m + 1, e);
    }
}

int main()
{

    int n;
    while(scanf("%d", &n) != EOF)
    {
        for(int i = 0; i < n; i ++)
        {
            scanf("%d",&a[i]);
        }
        sum = 0;
        Merge_sort(0,n-1);
        printf("%lld\n",sum);
    }
    return 0;
}