':type t1: TreeNode
:type t2: TreeNode
:rtype: TreeNode'

def mergeTrees(self, t1, t2):

    if (t1 is None):
        return t2
    if (t2 is None):
        return t1
    t1.val += t2.val
    t1.left = self.mergeTrees(t1.left, t2.left)
    t1.right = self.mergeTrees(t1.right, t2.right)
    return t1
