Leetcode -- 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 ).

For example:
Given Binary Tree{3,9,20,#,#,15,7},

    3   /   9  20    /     15   7

Return its level order traversal:

[  [3],  [9,20],  [15,7]]

Original question link: https://oj.leetcode.com/problems/binary-tree-level-order-traversal/

Question: Given a binary tree, return the sequence traversal value of the node. (For example, from left to right, layer by layer)

Idea: similar to the breadth-first traversal, nodes can be placed in the queue in sequence, and then retrieved by layer.

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