Submission #24773656

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Source Code Expand

#include "bits/stdc++.h"

#define int long long

using namespace std;
using ll = long long;
using P = pair<ll, ll>;
const ll INF = (1LL << 61);
ll mod = 1000000007;
struct mint {
	ll x; // typedef long long ll;
	mint(ll x = 0) :x((x%mod + mod) % mod) {}
	mint operator-() const { return mint(-x); }
	mint& operator+=(const mint a) {
		if ((x += a.x) >= mod) x -= mod;
		return *this;
	}
	mint& operator-=(const mint a) {
		if ((x += mod - a.x) >= mod) x -= mod;
		return *this;
	}
	mint& operator*=(const mint a) {
		(x *= a.x) %= mod;
		return *this;
	}
	mint operator+(const mint a) const {
		mint res(*this);
		return res += a;
	}
	mint operator-(const mint a) const {
		mint res(*this);
		return res -= a;
	}
	mint operator*(const mint a) const {
		mint res(*this);
		return res *= a;
	}
	mint pow(ll t) const {
		if (!t) return 1;
		mint a = pow(t >> 1);
		a *= a;
		if (t & 1) a *= *this;
		return a;
	}

	// for prime mod
	mint inv() const {
		return pow(mod - 2);
	}
	mint& operator/=(const mint a) {
		return (*this) *= a.inv();
	}
	mint operator/(const mint a) const {
		mint res(*this);
		return res /= a;
	}
};
istream& operator>>(istream& is, mint& a) { return is >> a.x; }
ostream& operator<<(ostream& os, const mint& a) { return os << a.x; }

int N;
char c[100010];
vector<int>G[100010];
mint dp[100010][3];
void dfs(int v, int p = -1) {
	mint S = 1, T = 1;
	if (c[v] == 'a') {
		for (auto nv : G[v]) {
			if (nv == p)continue;
			dfs(nv, v);
			S *= (dp[nv][0] + dp[nv][2]);
			T *= (dp[nv][0] + dp[nv][1] + (mint)2 * dp[nv][2]);
		}
		dp[v][0] = S;
		dp[v][2] = T - S;
	}
	else {
		for (auto nv : G[v]) {
			if (nv == p)continue;
			dfs(nv, v);
			S *= (dp[nv][1] + dp[nv][2]);
			T *= (dp[nv][0] + dp[nv][1] + (mint)2 * dp[nv][2]);
		}
		dp[v][1] = S;
		dp[v][2] = T - S;
	}
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cin >> N;
	for (int i = 0; i < N; i++)cin >> c[i];
	for (int i = 0; i < N - 1; i++) {
		int a, b; cin >> a >> b; a--; b--;
		G[a].push_back(b);
		G[b].push_back(a);
	}
	dfs(0, -1);
	cout << dp[0][2] << endl;
	return 0;


}

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