107. Binary Tree Level Order Traversal II(层序遍历)

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]
]

# 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]]
        """
        list = []
        result = []
        if not root:
            return result
        list.append(root)
        while len(list):
            temp = []
            for i in range(len(list)):
                current = list.pop(0)
                temp.append(current.val)
                if current.left:
                   list.append(current.left)
                if current.right:
                 list.append(current.right)
            result.append(temp)
        return result[::-1]

不要忘记判断左右节点是否为空