Submission #3772957

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;
using Int = long long;
struct node {
    vector<int> next;
};
int N;
vector<node> graph;
vector<int> memo;
const int MOD = 1000000007;
int dfs(int i, bool c, int prev = -1) // c: true == black
{
    if (memo[c*N + i] > 0) return memo[c*N + i];
    Int ans = 1;
    for (int n : graph[i].next) {
        if (n != prev) {
            if (c) {
                ans *= dfs(n, false, i);
            } else {
                ans *= dfs(n, false, i) + dfs(n, true, i);
            }
            ans %= MOD;
        }
    }
    return memo[c*N + i] = ans;
}
int main()
{
    cin >> N;
    graph = vector<node>(N);
    memo = vector<int>(N*2);
    for (int i = 0; i < N-1; i++) {
        int a, b; cin >> a >> b;
        a--, b--;
        graph[a].next.push_back(b);
        graph[b].next.push_back(a);
    }
    cout << (dfs(0, false) + dfs(0, true)) % MOD << endl;
    return 0;
}

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