归并找逆序对

#include<iostream>
using namespace std;

const int maxn = 1000;
int a[maxn],t[maxn];
int cnt=0;

void marge_sort(int a[],int x,int y,int t[]) //参数时0~n的不是0~n-1
{
    if(y-x>1)
    {   int m = x + (y-x)/2; //求中值,曾错过，是y-x不是x+y
        int q =x,p=m,i = x; //
        marge_sort(a,x,m,t);//递归求解
        marge_sort(a,m,y,t);
        while(q<m || p<y) //将两个数组，存到按序t数组中
        {    //cout<<"++";
            if(p >=y || (q < m && a[q]<=a[p])) t[i++]=a[q++];
            else
            { //  cout<<cnt;
                t[i++] = a[p++];
                cnt +=  m -q;
            }
        }
        for(int i = x;i < y;i++)
            a[i] = t[i];
    }
}



int main()
{
    int n;
    cin>>n;
    for(int i = 0;i < n;i ++ )       cin>>a[i];

    marge_sort(a,0,n,t);
   // for(int i  = 0 ;i < n ;i++)
    //    cout<<a[i]<<" ";
   // cout<<endl;
    cout<<cnt<<endl;

    return 0;
}