Basic Recursive BFS

RecursiveBFS.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 levelOrder(self, root: TreeNode) -> List[List[int]]:
        
        levels = collections.defaultdict(list)

        def bfs(node, level):
            if node:
                levels[level].append(node.val)
                bfs(node.left, level+1)
                bfs(node.right, level+1)
        
        bfs(root, 0)
        
        return levels.values()