Top View of Binary Tree

import java.util.*;

class Node {
    int data;
    Node left, right;

    Node(int val) {
        this.data = val;
        this.left = null;
        this.right = null;
    }
}

class Solution {
    public ArrayList<Integer> topView(Node root) {
        ArrayList<Integer> ans = new ArrayList<>();
        if (root == null)
            return ans;

        Map<Integer, Integer> map = new TreeMap<>();
        Queue<Pair> q = new LinkedList<>();
        q.add(new Pair(root, 0));

        while (!q.isEmpty()) {
            Pair p = q.poll();
            Node node = p.node;
            int hd = p.hd;

            if (!map.containsKey(hd))
                map.put(hd, node.data);

            if (node.left != null)
                q.add(new Pair(node.left, hd - 1));
            if (node.right != null)
                q.add(new Pair(node.right, hd + 1));
        }

        for (int val : map.values())
            ans.add(val);

        return ans;
    }
}

class Pair {
    Node node;
    int hd;

    Pair(Node n, int h) {
        node = n;
        hd = h;
    }
}