leetcode-102. Binary Tree Level Order Traversal

题目：

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 7
return its level order traversal as:
[
[3],
[9,20],
[15,7]
]

层次遍历，用FIFO比较合适。
新建一个FIFO队列引用，为每一层维护一个FIFO队列然后遍历这个队列，同时添加当前元素和将当前元素的左右子节点分别添加到下一层FIFO队列中。

/** * 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>> ret = new ArrayList<List<Integer>>();        List<TreeNode> tmp = new ArrayList<TreeNode>();        if(root!=null) tmp.add(root);        while(tmp.size()!=0){            List<Integer> ad = new ArrayList<Integer>();            ArrayList<TreeNode> replace = new ArrayList<TreeNode>();            for(TreeNode node : tmp){                if(node.left!=null)                    replace.add(node.left);                if(node.right!=null)                    replace.add(node.right);                ad.add(node.val);            }            tmp = replace;            ret.add(ad);        }        return ret;    }}