Binary Tree Level Order Traversal

Medium

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

For 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 of a Binary Tree

The TreeNode class is represented as:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */

Implementation

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        
        List<List<Integer>> result = new ArrayList<>();
        if(root == null){
            return result;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            int size = queue.size();
            List<Integer> level = new ArrayList<>();
            for(int i = 0 ; i < size ; i++){
                TreeNode curNode = queue.poll();
                level.add(curNode.val);
                if(curNode.left != null){
                    queue.offer(curNode.left);
                }
                if(curNode.right != null){
                    queue.offer(curNode.right);
                }
            }
            result.add(level);
        }
        return result;
    }
}

Categories: Binary Tree

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