uva 1428 - Ping pong

树状数组初级用法：

//#pragma comment(linker, "/STACK:1024000000,1024000000")
#include <iostream>
#include <stack>
#include <queue>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <set>
#include <vector>
#include <cstring>
#include <algorithm>

#define INF 0x3fffffff
#define inf -0x3f3f3f3f
#define N 50010
#define M 100010
#define LL long long
#define mod 95041567

using namespace std;

int n, MAX;
int arr[N], d[M], p[N], q[N];

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

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

void add(int x){
    while(x <= MAX){
        ++ d[x];
        x += lowbit(x);
    }
}

int main() {
    //freopen("in.txt", "r", stdin);
    int n, t;
    scanf("%d", &t);
    while(t --){
        scanf("%d", &n);
        MAX = 0;
        for(int i = 0; i < n; ++ i){
            scanf("%d", &arr[i]);
            MAX = max(MAX, arr[i]);
            p[i] = q[i] = 0;
        }
        memset(d, 0, sizeof(d));
        for(int i = 0; i < n; ++ i){
            add(arr[i]);
            p[i] = sum(arr[i] - 1);
        }
        memset(d, 0, sizeof(d));
        for(int i = n - 1; i >= 0; -- i){
            add(arr[i]);
            q[i] = sum(arr[i] - 1);
        }
        LL cnt = 0;
        for(int i = 0; i < n; ++ i){
            cnt += (LL)p[i] * (n - i - 1 - q[i]) + (LL)(i - p[i]) * q[i];
        }
        //cout<< cnt <<endl;
        printf("%lld\n", cnt);
    }
    return 0;
}