```
# 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]]
"""
if root == None: return []
res, cur_level = [], [root]
while cur_level:
res.append([x.val for x in cur_level])
next_level = []
for x in cur_level:
if x.left != None: next_level.append(x.left)
if x.right != None: next_level.append(x.right)
cur_level = next_level
return res
```