I get a lot of runtime errors. I get so frustrated. So finally, I just add the following code:

```
class Solution:
# @param root, a tree node
# @return an integer
def maxPathSum(self, root):
if root == None:
return 0
def dfs(node):
if node == None: return
if node.left:
dfs(node.left)
if node.right:
dfs(node.right)
dfs(root)
```

It still gives me runtime error. I would expect at least a wrong answer. Anyone knows why?