Binary Tree Level Order Traversal - The Coding Shala

Last Updated: 19-Jan-2021
Home >> Data Structures >> Binary Tree Level Order Traversal

 In this post, we will learn how to Traverse a Binary Tree in Level Order.

Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

Example:
Given binary tree [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
return its level order traversal as:
[
  [3],
  [9,20],
  [15,7]
]

Level Order Traversal of Binary Tree in Java

Approach 1

Using Queue[BFS]. Iterative Solution.

Java Program: 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ans = new ArrayList<>();
        if(root == null) return ans;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            int size = queue.size();
            List<Integer> tmp = new ArrayList<>();
            for(int i=0; i< size; i++){
                TreeNode curr = queue.poll();
                tmp.add(curr.val);
                if(curr.left != null) queue.offer(curr.left);
                if(curr.right != null) queue.offer(curr.right);
            }
            ans.add(tmp);
        }
        return ans;
    }
}


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