TopViewBinaryTree

// Java program to print top view of Binary tree
import java.util.*;
import java.util.Map.Entry;
                                                                                      //Time Complexity: O(n)
// Class for a tree node
class TreeNode
{
	// Members
	int key;
	TreeNode left, right;
    int hd;
	// Constructor
	public TreeNode(int key)
	{
		this.key = key;
		left = right = null;
		hd = Integer.MAX_VALUE;
	}
}

class Tree
{
	TreeNode root;

	public Tree() { root = null; }
	public Tree(TreeNode n) { root = n; }

	public void printTopView()
	{
		// base case
		if (root == null) { return; }
        int hd = 0;
        Map<Integer, Integer> map = new TreeMap<>();
        
        root.hd =hd;
		Queue<TreeNode> Q = new LinkedList<TreeNode>();
		Q.add(root); // Horizontal distance of root is 0

		// Standard BFS or level order traversal loop
		while (!Q.isEmpty())
		{
			TreeNode n = Q.remove();
			if (!map.containsKey(n.hd))
			{
				map.put(n.hd,n.key);
				
			}

			// Enqueue left and right children of current node
			if (n.left != null)
            {
                n.left.hd = n.hd-1;
                Q.add(n.left);
            }
            // If the dequeued node has a left child add it to the
            // queue with a horizontal distance hd+1.
            if (n.right != null)
            {
                n.right.hd = n.hd+1;
                Q.add(n.right);
            }
		}

		Set<Entry<Integer, Integer>> set = map.entrySet();
         Iterator<Entry<Integer, Integer>> iterator = set.iterator();
         while (iterator.hasNext())
        {
            Map.Entry<Integer, Integer> me = iterator.next();
            System.out.print(me.getValue()+" ");
        }
	}
}

public class Main
{
	public static void main(String[] args)
	{
		TreeNode root = new TreeNode(1);
		root.left = new TreeNode(2);
		root.right = new TreeNode(3);
		root.left.left = new TreeNode(4);
		root.left.right = new TreeNode(5);
		root.right.right = new TreeNode(7);
		root.right.left = new TreeNode(6);
		Tree t = new Tree(root);
		System.out.println("Following are nodes in top view of Binary Tree");
		t.printTopView();
	}
}