107. Binary Tree Level Order Traversal II

题目

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]
]

myanswer:

只需在102. Binary Tree Level Order Traversal改动一行即可

class Solution:
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root:
            return []
        res, level = [],[root]
        while level:
            currentNode = []
            nextLevel = []
            for node in level:
                currentNode.append(node.val)
                if node.left:
                    nextLevel.append(node.left)
                if node.right:
                    nextLevel.append(node.right)
            res.insert(0, currentNode)   # 每次都给插入到列表的头部
            level = nextLevel
        return res