107. 二叉树的层次遍历 II

给定一个二叉树，返回其节点值自底向上的层次遍历。 （即按从叶子节点所在层到根节点所在的层，逐层从左向右遍历）

例如：
给定二叉树 [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

返回其自底向上的层次遍历为：

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

思路：
与102层次遍历类似，逆序添加到结果列表中即可。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        
        if root == None:
            return []
        
        result, temp, queue = [], [], []
        
        current_layer_last, next_layer_last = root, root
        
        queue.append(root)
        
        while queue:
            curr_node = queue.pop(0)
            temp.append(curr_node.val)
            
            curr_node_left, curr_node_right = curr_node.left, curr_node.right
            
            if curr_node_left:
                queue.append(curr_node_left)
                next_layer_last = curr_node_left
        
            if curr_node_right:
                queue.append(curr_node_right)
                next_layer_last = curr_node_right
                
            if curr_node == current_layer_last:
                current_layer_last =next_layer_last
                result.insert(0, temp)
                temp = []
                
        return result