':type l1: ListNode
:type l2: ListNode
:rtype: ListNode'

def mergeTwoLists(self, l1, l2):

    curr = dummy = ListNode(0)
    while (l1 and l2):
        if (l1.val < l2.val):
            curr.next = l1
            l1 = l1.next
        else:
            curr.next = l2
            l2 = l2.next
        curr = curr.next
    curr.next = (l1 or l2)
    return dummy.next
