The idea is simple, when `t1`

and `t2`

are both `None`

, return `None`

otherwise just turtle all the way down.

```
class Solution(object):
def mergeTrees(self, t1, t2):
"""
:type t1: TreeNode
:type t2: TreeNode
:rtype: TreeNode
"""
if not t1 and not t2: return None
if t1:
v1, L1, R1 = t1.val, t1.left, t1.right
else:
v1, L1, R1 = 0, None, None
if t2:
v2, L2, R2 = t2.val, t2.left, t2.right
else:
v2, L2, R2 = 0, None, None
node = TreeNode(v1+v2)
node.left = self.mergeTrees(L1, L2)
node.right = self.mergeTrees(R1, R2)
return node
```