问题链接

https://leetcode.com/explore/interview/card/top-interview-questions-easy/94/trees/628/

解题思路

1、先序遍历
2、每次记录层次，作为子数组的index
3、将每次的root节点append到指定层次的子数组中

具体代码

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

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = []
        self.preorder(res, 0, root)
        return res
        
    def preorder(self, res, level, root):
        if root:
            if len(res) < level + 1:
                res.append([])
            
            res[level].append(root.val)
            
            self.preorder(res, level + 1, root.left)
            self.preorder(res, level + 1, root.right)