Witch game

#include <iostream>
#include <set>
using namespace std;

int constexpr N = 1e5 + 10;

bool cycle3[N];
int child[N];
set<int> adj[N];

long long bn2(long long n) {
	return n*(n-1)/2;
}

int main() {
	int n, m = 0;
	cin >> n;

	for (int i = 0 ; i != n ; ++i) {
		int x;
		cin >> x; --x;
		adj[x].emplace(i);
		adj[i].emplace(x);
		child[i] = x;
	}

	for(int v = 0 ; v != n ; ++v)
		m += adj[v].size();
	m /= 2;

	for(int v = 0 ; v != n ; ++v)
		cycle3[v] = child[v] > v && child[child[v]] > v && adj[v].find(child[child[v]]) != adj[v].end();

	long long sum = 0;

	for(int v = 0 ; v != n ; ++v) {
		int deg = adj[v].size();
		int deg2 = 0;

		for(auto const & s : adj[v])
			deg2 += adj[s].size();

		deg2 -= cycle3[v];

		sum += bn2(n - v - 1 - deg) - (m - deg2);

		for(auto const & s : adj[v])
			adj[s].erase(v);
		m -= deg;
	}

	cout << sum;
}

Test 1

Group: 1

Verdict: ACCEPTED

Test 2

Group: 1

Verdict: ACCEPTED

Test 3

Group: 1

Verdict: ACCEPTED

Test 4

Group: 1

Verdict: ACCEPTED

Test 5

Group: 1

Verdict: ACCEPTED

Test 6

Group: 2

Verdict: ACCEPTED

Test 7

Group: 2

Verdict: ACCEPTED

Test 8

Group: 2

Verdict: ACCEPTED

Test 9

Group: 2

Verdict: ACCEPTED

Test 10

Group: 2

Verdict: ACCEPTED

Test 11

Group: 3

Verdict: ACCEPTED

Test 12

Group: 3

Verdict: ACCEPTED

Test 13

Group: 3

Verdict: ACCEPTED

Test 14

Group: 3

Verdict: ACCEPTED

Test 15

Group: 3

Verdict: ACCEPTED