归并排序（分治）

#include<iostream>
#include<cstdio>
using namespace std;
typedef long long ll;
#define N 500010
ll a[N],b[N];
ll n,ans;
void merge_sort(ll x,ll y)
{
    if (y-x>1)
    {
        ll m=x+(y-x)/2;
        ll  p=x,q=m,i=x;
        merge_sort(x,m);
        merge_sort(m,y);
       while (p<m||q<y)
       {
           if (q>=y||(p<m&&a[p]<=a[q]))
            b[i++]=a[p++];
           else
           {
               if (p<m)
                ans+=(m-p);
                  b[i++]=a[q++];
           }

       }
       for (ll i=x;i<y;i++)
            a[i]=b[i];
    }
}
int main()
{
    while (cin>>n&&n!=0)
    {
        ans=0;
        for (ll i=0;i<n;i++)
            cin>>a[i];
            merge_sort(0,n);
            cout << ans << endl;
    }
    return 0;
}