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 Java (OpenJDK 11.0.6) (judge verdict: Wrong Answer)



import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.StringTokenizer;

public class Main {
	static ArrayList<Integer> [] adj;
	static long dp[][];
	static int m=1000000007;
	// dp[i][j] number of ways to print the i rooted tree with or without constriants 
	// there can be only two possibilites that parent of i is black than we have to print i with W otherwise we have no contriants
	
	public static long Solve(int u,int constraint,int parent) {
		
		if(dp[u][constraint]!=-1) return dp[u][constraint];
		
		long ans=0;
		long w1=1;
		for(int v:adj[u]) {
			if(v!=parent) {
				w1=(w1%m * Solve(v, 0, u)%m)%m;
			}
		}
		ans+=(w1%m);
		
		if(constraint==0) {
			long w2=1;
			for(int v:adj[u]) {
				if(v!=parent) {
					w2=(w2%m* Solve(v, 1, u)%m)%m;
				}
			}
			ans+=(w2%m);
		}
		
		
		dp[u][constraint]=ans;
		return dp[u][constraint];
		
		
	}
	

	public static void main(String[] args) {
		FastScanner sc=new FastScanner();
		int n=sc.nextInt();
		adj=new ArrayList[n+1];
		for(int i=0;i<=n;i++) adj[i]=new ArrayList<>();
		
		for(int i=0;i<n-1;i++) {
			int x=sc.nextInt(),y=sc.nextInt();
			adj[x].add(y);
			adj[y].add(x);
		}
		
		dp=new long[100005][2];
		for(int i=0;i<100005;i++) Arrays.fill(dp[i], -1);
		
		long ans=Solve(1, 0, -1);
		System.out.println(ans);

	}

	static class FastScanner{
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st=new StringTokenizer("");
		
		String next() {
			while (!st.hasMoreTokens())
				try {
					st=new StringTokenizer(br.readLine());
				} catch (IOException e) {
					e.printStackTrace();
				}
			return st.nextToken();
		}
		int nextInt() {
			return Integer.parseInt(next());
		}
		long nextLong() {
			return Long.parseLong(next());
		}
		int [] readArray(int n) {
			int a[]=new int[n];
			for(int i=0;i<n;i++) a[i]=nextInt();
			return a;
		}
		
		
	}
	

}