package DataStructures.Trees;// Java program to print top view of Binary tree

import java.util.HashSet;
import java.util.LinkedList;
import java.util.Queue;

// Class for a tree node
class TreeNode {
 // Members
 int key;
 TreeNode left, right;

 // Constructor
 public TreeNode(int key) {
 this.key = key;
 left = right = null;
 }
}

// A class to represent a queue item. The queue is used to do Level
// order traversal. Every Queue item contains node and horizontal
// distance of node from root
class QItem {
 TreeNode node;
 int hd;

 public QItem(TreeNode n, int h) {
 node = n;
 hd = h;
 }
}

// Class for a Binary Tree
class Tree {
 TreeNode root;

 // Constructors
 public Tree() {
 root = null;
 }

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

 // This method prints nodes in top view of binary tree
 public void printTopView() {
 // base case
 if (root == null) {
 return;
 }

 // Creates an empty hashset
 HashSet<Integer> set = new HashSet<>();

 // Create a queue and add root to it
 Queue<QItem> Q = new LinkedList<QItem>();
 Q.add(new QItem(root, 0)); // Horizontal distance of root is 0

 // Standard BFS or level order traversal loop
 while (!Q.isEmpty()) {
 // Remove the front item and get its details
 QItem qi = Q.remove();
 int hd = qi.hd;
 TreeNode n = qi.node;

 // If this is the first node at its horizontal distance,
 // then this node is in top view
 if (!set.contains(hd)) {
 set.add(hd);
 System.out.print(n.key + " ");
 }

 // Enqueue left and right children of current node
 if (n.left != null)
 Q.add(new QItem(n.left, hd - 1));
 if (n.right != null)
 Q.add(new QItem(n.right, hd + 1));
 }
 }
}

// Driver class to test above methods
public class PrintTopViewofTree {
 public static void main(String[] args) {
 /* Create following Binary Tree
 1
 / \
 2 3
 \
 4
 \
 5
 \
 6*/
 TreeNode root = new TreeNode(1);
 root.left = new TreeNode(2);
 root.right = new TreeNode(3);
 root.left.right = new TreeNode(4);
 root.left.right.right = new TreeNode(5);
 root.left.right.right.right = new TreeNode(6);
 Tree t = new Tree(root);
 System.out.println("Following are nodes in top view of Binary Tree");
 t.printTopView();
 }
}