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 7return its level order traversal as:[ [3], [9,20], [15,7]]
难度：medium
题目：给定二叉树，返回其层次遍历结点值。
思路：队列(FIFO)
Runtime: 1 ms, faster than 86.63% of Java online submissions for Binary Tree Level Order Traversal.
Memory Usage: 37.5 MB, less than 100.00% of Java online submissions for 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; } * } */class Solution { public List > levelOrder(TreeNode root) { List > result = new ArrayList<>(); if (null == root) { return result; } Queue queue = new LinkedList<>(); queue.add(root); int currentLevelCount = 1, nextLevelCount = 0; List currentLevelElements = new ArrayList<>(); while (!queue.isEmpty()) { TreeNode node = queue.poll(); currentLevelElements.add(node.val); if (node.left != null) { queue.add(node.left); nextLevelCount++; } if (node.right != null) { queue.add(node.right); nextLevelCount++; } currentLevelCount--; if (0 == currentLevelCount) { result.add(currentLevelElements); currentLevelElements = new ArrayList<>(); currentLevelCount = nextLevelCount; nextLevelCount = 0; } } return result; }}