L3：08 - Binary Tree Level Order Traversal

https://oj.leetcode.com/problems/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 as: [ [3], [9,20], [15,7] ]

/** * Copyright: NineChapter * - Algorithm Course, Mock Interview, Interview Questions * - More details on: http://www.ninechapter.com/ */public class Solution {    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {        ArrayList result = new ArrayList();        if (root == null) {            return result;        }        Queue<TreeNode> queue = new LinkedList<TreeNode>();        queue.offer(root);        while (!queue.isEmpty()) {            ArrayList<Integer> level = new ArrayList<Integer>();            int size = queue.size();            for (int i = 0; i < size; i++) {                TreeNode head = queue.poll();                level.add(head.val);                if (head.left != null) {                    queue.offer(head.left);                }                if (head.right != null) {                    queue.offer(head.right);                }            }            result.add(level);        }        return result;    }}