题目描述

blablabla

样例

#include<iostream>
using namespace std;

//定义long long 类型为LL
typedef long long LL;

const int N = 100010;
int q[N],tmp[N];

LL merge_sort(int q[], int l, int r)
{
    if(l >= r) return 0;

    int mid = l + r >> 1 // '+'运算顺序要高于'>>'
    int i = l, j = mid + 1, k = 0;

    //此时的res是mid左右两侧的数的逆序对数量之和
    LL res = merge_sort(q, l, mid) + merge_sort(q, mid + 1, r);

    while(i <= mid && j <= r)
    {
        if(q[i] <= q[j])  tmp[k++] = q[i++];
        else
        {
            tmp[k++] = q[j++];
            //mid-i+1是当逆序对的较大的数在mid左侧，较小的数在mid
            //右侧时的逆序对数量
            res += mid - i + 1;
        }
    }

    //将剩下的数传到tmp数组中
    while(i <= mid) tmp[k++] = q[i++];
    while(j <= r) tmp[k++] = q[j++];

    //将tmp的值传回给q数组，防止递归时少数。
    for(int i = l, j = 0; i <= r, j < k; i++, j++)
    {
        q[i] = tmp[k];
    }

    //返回res逆序对的数量
    return res;
}

int main()
{
    int n;
    scanf("%d",&n);

    for(int i = 0; i < n; i++)
    {
        scanf("%d",&q[i]);
    }

    //逆序对数量
    LL res = merge_sort(q, 0, n - 1);

    cout << res << endl;

    return 0;
}