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]
]

题目大意

按层序从上到下遍历一颗树。

解题思路

用一个队列即可实现。