Binary Tree Level Order Traversal

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