Java Data Structure Binary Tree : Print The Nth Level

Printing Nth Level in a binary tree is the same as doing level order traversal.

Logic : 
We loop through all the levels. And, we maintain the current level count.
If the level count is the same as the level requested by the user then print the level information.

You can watch me code the solution on youtube.

Here is the code which prints the Nth level.

private static void printNthLevel(final BinaryTreeNode root, final int level){
        if(root == null) return;

    final BinaryTreeNode markForLevelEnd = new BinaryTreeNode(-1);

    final Deque<BinaryTreeNode> queue = new ArrayDeque<>();
    queue.addLast(root);
    queue.addLast(markForLevelEnd);

    int currentLevel = 1;
    while(!queue.isEmpty()){
         if(currentLevel > level) break;

         final BinaryTreeNode currentTreeNode = queue.removeFirst();

         if(currentTreeNode == markForLevelEnd){
             ++currentLevel;
         if(!queue.isEmpty()) { queue.addLast(markForLevelEnd); }
         continue;
         }

         if(currentLevel == level) {
         System.out.format("Level Number %d ", currentLevel);
             printBinaryTreeNode(currentTreeNode);
          }

         if(currentTreeNode.left  != null) queue.addLast(currentTreeNode.left);
         if(currentTreeNode.right != null) queue.addLast(currentTreeNode.right);

    }
    }

In the above code, I have queue, which holds all the nodes at a given level.

I have a currentLevel, which stores the current level count. At line number 12, I check, that if currentLevel is greater than the required level, if that is true , then I break out of the while-loop to stop the execution.

If currentLevel  is the same as level , then we print the information.

We put null  as marker to show end of a level. When we find a null  value, we increase the level count.

You can read my source code on github, for better reading experience

Full Source Code:

import java.util.Deque;
import java.util.ArrayDeque;

public class PrintNthLevel{
    private static void printBinaryTreeNode(final BinaryTreeNode node){
        if(node == null) System.out.println("NULL Node");

    System.out.format("[%d] ", node.data);
    }

    private static class BinaryTreeNode{
        private int data;
        private BinaryTreeNode left;
        private BinaryTreeNode right;
    
        private BinaryTreeNode(final int data, final BinaryTreeNode left, final BinaryTreeNode right){
            this.data  = data;
    	this.left  = left;
    	this.right = right;
        }
        private BinaryTreeNode(final int data){
            this(data, null, null);
        }
    }
    
    public static void main(final String[] args){
        final BinaryTreeNode one   = new BinaryTreeNode(1);
        final BinaryTreeNode two   = new BinaryTreeNode(2);
        final BinaryTreeNode three = new BinaryTreeNode(3);
        final BinaryTreeNode four  = new BinaryTreeNode(4);
        final BinaryTreeNode five  = new BinaryTreeNode(5);
        final BinaryTreeNode six   = new BinaryTreeNode(6);
    
        four.left = one;
        four.right = five;
    
        five.right = six;
    
        one.left = two;
        one.right = three;

    printNthLevel(four, 1);
    System.out.println();

    printNthLevel(four, 2);
    System.out.println();

    printNthLevel(four, 3);
    System.out.println();

    printNthLevel(four, 4);
    System.out.println();
    }

    private static void printNthLevel(final BinaryTreeNode root, final int level){
        if(root == null) return;

    final BinaryTreeNode markForLevelEnd = new BinaryTreeNode(-1);

    final Deque<BinaryTreeNode> queue = new ArrayDeque<>();
    queue.addLast(root);
    queue.addLast(markForLevelEnd);

    int currentLevel = 1;
    while(!queue.isEmpty()){
         if(currentLevel > level) break;

         final BinaryTreeNode currentTreeNode = queue.removeFirst();

         if(currentTreeNode == markForLevelEnd){
             ++currentLevel;
         if(!queue.isEmpty()) { queue.addLast(markForLevelEnd); }
         continue;
         }

         if(currentLevel == level) {
         System.out.format("Level Number %d ", currentLevel);
             printBinaryTreeNode(currentTreeNode);
          }

         if(currentTreeNode.left  != null) queue.addLast(currentTreeNode.left);
         if(currentTreeNode.right != null) queue.addLast(currentTreeNode.right);

    }
    }

}