leetcode: 21. Merge Two Sorted Lists

# Merge two sorted linked lists and return it as a new list.
# The new list should be made by splicing together the nodes of the first two lists.

class ListNode():
    def __init__(self, x):
        self.val = x
        self.next = None

class Solution():
    def mergeTwoLists(self, l1, l2):
        cur = dummy = ListNode(0)
        while l1 and l2:
            if l1.val <= l2.val:
                cur.next, l1 = l1, l1.next
            else:
                cur.next, l2 = l2, l2.next
            cur = cur.next
        cur.next = l1 or l2
        return dummy.next


if __name__ == "__main__":
    l1, l1.next = ListNode(0), ListNode(3)
    l2, l2.next = ListNode(1), ListNode(2)
    result = Solution().mergeTwoLists(l1, l2)
    assert '{0}->{1}->{2}->{3}'.format(result.val, result.next.val, result.next.next.val, result.next.next.next.val) \
           == '0->1->2->3'