Tree matching

#include <iostream>
#include <algorithm>
#include <cstdio>

using namespace std;

using ll = long long;

ll modulo = 1e9 + 7;

vector< pair<int, int> > v;

pair<ll, ll> calculate(int node) {
    if (v[node].first == -1 && v[node].second == -1) {
        return make_pair(0, 1);
    }

    pair<ll, ll> left = make_pair(0, 0);

    if (v[node].first != -1)
        left = calculate(v[node].first);

    pair<ll, ll> right = make_pair(0, 0);

    if (v[node].second != -1)
        right = calculate(v[node].second);

    ll a1 = left.first; // root included
    ll b1 = left.second; // root excluded

    ll a2 = right.first;
    ll b2 = right.second;

    ll included = 0;
    ll excluded = 0;

    if (v[node].first == -1) {
        excluded = (a2 + b2) % modulo;
        included = b2;
    } else if (v[node].second == -1) {
        excluded = (a1 + b1) % modulo;
        included = b1;
    } else {
        excluded = ((a1 + b1) * (a2 + b2)) % modulo;
        included = (((b1) * (a2 + b2)) % modulo + ((a1 + b1) * (b2)) % modulo) % modulo;
    }

//    cout << node <<  " " << included << " " << excluded << endl;

    return make_pair(included, excluded);
}

int main(int argc, char *argv[])
{
    int n;
    cin >> n;

    for (int i = 0; i < n; ++i) {
        int a, b;
        cin >> a >> b;

        v.push_back(make_pair(a - 1, b - 1));
    }

    pair<ll, ll> res = calculate(0);
    cout << (res.first + res.second) % modulo << endl;

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED