acwing 241. 楼兰图腾（树状数组）

题意

思路

1. 树状数组，我们通过枚举顺序进行插入，然后对树状数组进行区间查询可以了。

代码

#include <bits/stdc++.h>
using namespace std;

#define ll long long
const int N = 200005;
int n, a[N], c[N], great[N], lower[N];

int lowbit(int x)
{
    
    
    return x & -x;
}

void insert(int x, int y)
{
    
    
    while (x <= n)
    {
    
    
        c[x] += y;
        x += lowbit(x);
    }
}

int sum(int x)
{
    
    
    int all = 0;
    while (x > 0)
    {
    
    
        all += c[x];
        x -= lowbit(x);
    }
    return all;
}


int main()
{
    
    
    scanf("%d", &n);
    for (int i = 1; i <= n; i ++) scanf("%d", &a[i]);

    for (int i = 1; i <= n; i ++) {
    
    
        great[i] = sum(n) - sum(a[i]);
        lower[i] = sum(a[i] - 1);
        insert(a[i], 1);
    }

    memset(c, 0, sizeof c);
    ll res1 = 0, res2 = 0;
    for (int i = n; i; i --) {
    
    
        res1 += 1LL * great[i] * (sum(n) - sum(a[i]));
        res2 += 1LL * lower[i] * sum(a[i] - 1);
        insert(a[i], 1);
    }

    printf("%lld %lld\n", res1, res2);


    return 0;
}