def addNode(self, node1, node2):
        if not node1 and not node2:
            raise Exception("two nodes are None")
        if not node1:
            return node2.val
        if not node2:
            return node1.val
        return node1.val+node2.val
Handles None situation
dataset/ETHPy150Open algorhythms/LeetCode/004 Add Two Numbers.py/Solution.addNode