Sorting

input/code.cpp: In member function 'void fen::add(int)':
input/code.cpp:9:13: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(; pos < a.size(); pos += pos&-pos) a[pos]++;
         ~~~~^~~~~~~~~~

#include<bits/stdc++.h>
#define all(x) x.begin(), x.end()
using namespace std;
using ll = long long;
struct fen {
	vector<int> a;
	fen(int n) : a(n+1) {}
	void add(int pos) {
		for(; pos < a.size(); pos += pos&-pos) a[pos]++;
	}
	int get(int pos) {
		int res = 0;
		for(; pos; pos -= pos&-pos) res += a[pos];
		return res;
	}
};
int main() {
	cin.tie(0)->sync_with_stdio(0);
	int t;
	cin >> t;
	while(t--) {
		int n;
		cin >>n;
		vector<int> a(n);
		fen f(n);
		ll inv = 0;
		for(auto &i : a) {
			cin >> i;
			inv += f.get(i);
			f.add(i);
		};
		inv = n*1ll*(n-1)/2 - inv;
		if(n < 4) {
			inv = inv != 0;
		}
		if(n == 4) {
			inv = inv != 0 && inv != 2;
		}
		cout << (inv%2?"NO":"YES") << '\n';
	}
}

Test 1

Group: 1, 2

Verdict: WRONG ANSWER

Test 2

Group: 2

Verdict: WRONG ANSWER

Test 3

Group: 1, 2

Verdict: ACCEPTED

Test 4

Group: 1, 2

Verdict: ACCEPTED