[LeetCode]107 二叉树的层次遍历II

题目描述

给定一个二叉树，返回其按层次遍历的节点值。 （即逐层地，从左到右访问所有节点）。

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]
]

代码

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# 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 []
        stacks = []
        new_stack = [root]
        stacks.append(new_stack)
        
        while new_stack != []:
            new_stack_2 = []
            for i in new_stack:
                if i.left is None and i.right is None:
                    continue
                if i.left is not None:
                    new_stack_2.append(i.left)
                if i.right is not None:
                    new_stack_2.append(i.right)
            new_stack = new_stack_2
            stacks.append(new_stack)
        stacks.pop()
    
        results = []
        for stack in stacks:
            result = []
            for _ in stack:
                result.append(_.val)
            results.append(result)
        return results[::-1]