UVAlive - 4329 —— Ping pong 【树状数组】

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>

using namespace std;

const int MAXA = 100000 + 5;
int n;
int c[MAXA];

int sum(int i)
{
    int ret = 0;
    while(i > 0) {
        ret += c[i];
        i -= i & (-i);
    }    
    return ret;
}

void add(int i, int d)
{
    while(i <= MAXA-1) {
        c[i] += d;
        i += i & (-i);
    }
}

int main ()
{
    int T;
    int *a, *leftLess, *rightLess;
    scanf("%d", &T);
    while(T--) {
        scanf("%d", &n);
        a = new int[n];
        leftLess = new int[n];
        rightLess = new int[n];
        for(int i=0; i<n; i++) {
            scanf("%d", &a[i]);
            leftLess[i] = sum(a[i]-1);
            add(a[i], 1);
        }
        memset(c, 0, sizeof(c));
        for(int i=n-1; i>=0; i--) {
            rightLess[i] = sum(a[i]-1);
            add(a[i], 1);
        }
        long long ret = 0;
        for(int i=0; i<n; i++) {
            // a[i] is distinct integers, so the opsite side of leftLess is leftGreater
            ret += leftLess[i] * 1LL * (n-1-i-rightLess[i]);
            ret += (i-leftLess[i]) * 1LL * rightLess[i];
        }
        printf("%lld\n", ret);
        memset(c, 0, sizeof(c));
        delete(a);
        delete(leftLess);
        delete(rightLess);
    }
    
    return 0;
}