Problem:
Score : 100 points

Problem Statement

There is a tree with N vertices, numbered 1, 2, \ldots, N.
For each i (1 \leq i \leq N - 1), the i-th edge connects Vertex x_i and y_i.

Taro has decided to paint each vertex in white or black.
Here, it is not allowed to paint two adjacent vertices both in black.

Find the number of ways in which the vertices can be painted, modulo 10^9 + 7.

Constraints

All values in input are integers.

1 \leq N \leq 10^5

1 \leq x_i, y_i \leq N

The given graph is a tree.

Input

Input is given from Standard Input in the following format:

N
x_1 y_1
x_2 y_2
:
x_{N - 1} y_{N - 1}

Output

Print the number of ways in which the vertices can be painted, modulo 10^9 + 7.

Sample Input 1

3
1 2
2 3

Sample Output 1

5

There are five ways to paint the vertices, as follows:

Sample Input 2

4
1 2
1 3
1 4

Sample Output 2

9

There are nine ways to paint the vertices, as follows:

Sample Input 3

1

Sample Output 3

2

Sample Input 4

10
8 5
10 8
6 5
1 5
4 8
2 10
3 6
9 2
1 7

Sample Output 4

157

Solution in Java8 (OpenJDK 1.8.0) (judge verdict: Wrong Answer)


import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Scanner;
import java.util.Set;

public class Main {

	public static void main(String[] args) {
		Main m = new Main();
		m.run();
	}

	Scanner sc = new Scanner(System.in);

	void run() {
		int n = sc.nextInt();
		
		Map<Integer,List<Integer>> tree = new HashMap<Integer,List<Integer>>();
		
		for(int i = 0 ;i < n-1;i++){
			int x = sc.nextInt();
			int y = sc.nextInt();
			
			tree.computeIfAbsent(x, a->new ArrayList<Integer>()).add(y);
			tree.computeIfAbsent(y, a->new ArrayList<Integer>()).add(x);
		}
		
		int[] ans = search(tree,1, new HashSet<Integer>());
		
		System.out.println(ans[0]+ans[1]);
		
	}
	
	Map<Integer,int[]> memo = new HashMap<Integer,int[]>();
	int[] search(Map<Integer,List<Integer>> tree, int now, Set<Integer> set){
		if(memo.containsKey(now)){
			return memo.get(now);
		}
		
		set.add(now);
		
		int[] res = new int[2];
		
		int zero = 1;
		int all = 1;
		
		
		for(int next: tree.getOrDefault(now,new ArrayList<Integer>())){
			if(!set.contains(next)){
				int[] nv = search(tree,next,set);
				
				zero = prod(zero, nv[0]);
				all = prod(all,nv[0] + nv[1]);
			}
		}
		
		res[0] = all;
		res[1] = zero;

		memo.put(now, res);
		return res;
	}
	
	int mod = 1_000_000_000+7;
	int prod(int a, int b){
		return (int)(((long)((long)a % mod )* ((long)b % mod)) % mod);
	}
	
}