Day7 Binary Tree Level Order Traversal II

LeetCode107. Binary Tree Level Order Traversal II

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

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

比较理想的是按照层序把数字依次放进ans数组中，要层序遍历的话，需用到队列。

值得注意的是，应该让每次循环后，队列里都只留下二叉树的一整层。

全部放进数组后整个倒置一遍就ok了。