Binary Tree Level Order Traversal

/** * Definition for binary tree * 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>> result = new ArrayList<List<Integer>>();        if (root == null) return result;        Queue<TreeNode> queue = new LinkedList<TreeNode>();        queue.add(root);        while(!queue.isEmpty()){            int size = queue.size();            List<Integer> list = new ArrayList<Integer>();            result.add(list);            for (int i=0; i<size; i++){                TreeNode node = queue.remove();                list.add(node.val);                if (node.left != null){                    queue.add(node.left);                }                if (node.right != null){                    queue.add(node.right);                }            }        }        return result;    }}