Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level). Tags: Tree

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Solution

import collections

# 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 levelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:
            return []
        
        queue = collections.deque()
        queue.appendleft((root, 0))
        res = []
        
        while queue:
            top, level = queue.pop()
            
            if len(res) == level:
                res.append([])
            res[level].append(top.val)
            
            if top.left:
                queue.appendleft((top.left, level + 1))
            if top.right:
                queue.appendleft((top.right, level + 1))
        return res