https://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,null,null,15,7],

 3 / \ 9 20 / \ 15 7 

return its level order traversal as:

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

whilequeue不空： cur=queue.pop() for节点incur的所有相邻节点： if该节点有效且未访问过： queue.push(该节点)