Codeforces Round #790 (Div. 4) H1. Maximum Crossings (Easy Version)

• Time : O(nlogn)
• Space : O(n)

#include <bits/stdc++.h>

#pragma optimization_level 3
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx")
#pragma GCC optimize("Ofast")//Comment optimisations for interactive problems (use endl)
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization ("unroll-loops")

struct PairHash {inline std::size_t operator()(const std::pair<int,int> & v) const {return v.first*31+v.second;}};

#define MAX_N 1010
#define INF 987654321
#define EPS 1e-9
#define ll long long
#define pll pair<ll, ll>
#define vpll vector<pll>
#define vall3 vector<array<ll,3>>
#define all3 array<ll,3>
#define all5 array<ll,5>
#define vall5 vector<all5>
#define vll vector<ll>
#define vs vector<string>
#define usll unordered_set<ll>
#define uspll unordered_set<pll, PairHash>
#define vvs vector<vs>
#define vvll vector<vll>
#define all(a) begin(a), end(a)

using namespace std;

ll fenwick[MAX_N];

void update(ll n) {
    while(n < MAX_N) {
        fenwick[n] += 1;
        n += n & -n;
    }
}

ll query(ll n) {
    ll res = 0;
    while(n) {
        res += fenwick[n];
        n -= n & -n;
    }
    return res;
}

ll solve(vll& A) {
    ll n = A.size(), res = 0;
    memset(fenwick, 0, sizeof fenwick);
    for(ll i = n - 1; i >= 1; i--) {
        res += query(A[i]);
        update(A[i]);
    }
    return res;
}

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    ll tc = 1;
    cin>>tc;
    for(ll i = 1; i <= tc; i++) {
        ll n;
        cin>>n;
        vll A(n + 1);
        for(ll i = 1; i <= n; i++) cin>>A[i];
        cout<<solve(A)<<endl;
    }
    return 0;
}