Top view

data_structures/Trees/Print Top View of Tree.java

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+// Java program to print top view of Binary tree
+import java.util.*;
+ 
+// 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 Main
+{
+    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();
+    }
+}