Envío 6250

Código

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Scanner;
import java.util.Stack;

public class Main {

    private static HashMap<Integer, ArrayList<Integer>> parentSons = new HashMap<>();

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        for(int i=0; i<n-1; i++){
            int p = scanner.nextInt();
            int h = scanner.nextInt();

            if(!parentSons.containsKey(p)){
                ArrayList<Integer> arrayList = new ArrayList<Integer>();
                arrayList.add(h);
                parentSons.put(p, arrayList);
            } else {
                parentSons.get(p).add(h);
            }
        }

        int c = scanner.nextInt();
        for(int i=0; i<c; i++){
            int ancestor = scanner.nextInt();
            int descendant = scanner.nextInt();

            if(isAncestor(ancestor, descendant)){
                System.out.println("Yes");
            } else {
                System.out.println("No");
            }
        }
    }

    private static boolean isAncestor(int ancestor, int descendant){
        if(ancestor == descendant){
            return true;
        }

        if(descendant == 0){
            return false;
        }

        if(ancestor == 0){
            return true;
        }

        Stack<Integer> toProcess = new Stack<>();
        if(parentSons.containsKey(ancestor)) {
            toProcess.addAll(parentSons.get(ancestor));
        }

        while(!toProcess.isEmpty()){
            int node = toProcess.pop();

            if(descendant == node){
                return true;
            }

            if(parentSons.containsKey(node)){
                toProcess.addAll(parentSons.get(node));
            }
        }

        return false;
    }
}