归并排序——求逆序对

//归并排序及求逆序对
#include<bits/stdc++.h>
using namespace std;
#define N 1000005
int a[N],b[N]; //b为辅助数组
long long cnt;
void merge_sort(int l,int m, int r)
{

    int mid = m ;
    int i = l; //辅助数组的下标
    int p = l, q = mid+1;
    //printf("%d-%d  %d-%d\n",p,mid ,q ,r);
    while(p<=mid || q<=r)//左右两部分只要有一部分不为空
    {
        if(q>r || (p<=mid && a[p]<=a[q]))
        {//从左半数组复制到辅助数组
           b[i++] = a[p++];
        }

        else
        {
            b[i++] = a[q++];
            cnt += mid -p +1;//将逆序对的个数累加起来
        }
    }
    for(i = l ;i <= r;i++)//将b中排好序的元素复制到a中
        a[i] = b[i];
}
void Merge(int l,int r)
{
    if(l < r)
    {
        int m = (l + r) >> 1;
        Merge(l,m);
        Merge(m+1,r);
        merge_sort(l,m,r);
    }
}
int main()
{
    int n;
    while(cin >> n)
    {
        for(int i = 1 ; i <= n; i ++)
            cin >> a[i];
        cnt = 0;
        Merge(1, n);
        for(int i = 1; i <= n; i++)
            cout << a[i] << " ";
        cout << endl;
        printf("逆序对有：%d\n",cnt);
    }
    return 0;
}