JavaAlgorithms/DataStructures/Trees/PrintTopViewofTree.java

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();
  }
}