Skip to content

107. Binary Tree Level Order Traversal II

Difficulty Topics

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).

 

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: []

 

Constraints:

  • The number of nodes in the tree is in the range [0, 2000].
  • -1000 <= Node.val <= 1000

Solution

binary-tree-level-order-traversal-ii.py
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def levelOrderBottom(self, root):
        q, res = collections.deque([(root, 0)]), []

        while q:
            node, lvl = q.popleft()

            if node:
                if len(res) < lvl + 1:
                    res.insert(0, [])
                res[-(lvl+1)].append(node.val)
                for i in (node.left,node.right):
                    if i:
                        q.append([i, lvl+1])

        return res