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。