102. 二叉树的层次遍历

给定一个二叉树，返回其按层次遍历的节点值，即逐层从左到右访问所有节点

在子函数中传入层数，子树层数+1即可

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
	def __init__(self):
		self.res = []
		
	def levelOrder(self, root: TreeNode) -> List[List[int]]:

		def addLevel(tree: TreeNode, level: int):
			if tree is None:
				return 
			if len(self.res) < level:
				self.res.append([tree.val])
			else:
				self.res[level - 1].append(tree.val)
			addLevel(tree.left, level + 1)
			addLevel(tree.right, level + 1)
		
		addLevel(root, 1)
		return self.res