先遍历，记录下每个节点在第几层，然后按层输出。

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    # @param root, a tree node
    # @return a list of lists of integers
    def preOrder(self, root, level):
        if level in Solution.L:
            Solution.L[level].append(root.val)
        else:
            Solution.L[level] = [root.val]
        if root.left:
            self.preOrder(root.left, level + 1)
        if root.right:
            self.preOrder(root.right, level + 1)
        return
    
    def levelOrder(self, root):
        res = []
        if root == None: return res
        Solution.L = {}
        self.preOrder(root, 0)
        for i in sorted(Solution.L.keys()):
            res.append(Solution.L[i])
        return res