Leetcode 107. Binary Tree Level Order Traversal II

Description: Given the root of a binary tree, return the bottom-up level order traversal of its nodes' values. (i.e., from left to right, level by level from leaf to root).

Link: 107. Binary Tree Level Order Traversal II

Exaples:

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: [[15,7],[9,20],[3]]

Example 2:
Input: root = [1]
Output: [[1]]

Example 3:
Input: root = []
Output: []

思路: 因为是按照每一层返回，所以用广搜，BFS每一层的val，然后reverse list返回。

class Solution(object):
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if root is None: return []
        res = []
        que = collections.deque()
        que.append(root)
        while que:
            size = len(que)
            layerval = []
            for _ in range(size):
                node = que.popleft()
                if node is None:
                    continue
                layerval.append(node.val)
                que.append(node.left)
                que.append(node.right)
            if len(layerval)>0:
                res.append(layerval)
        return res[::-1]

日期: 2021-03-13 (Peer presure is so heavy, many papers are accepted in our group by NAACL, feel so stressed.)