21. Merge Two Sorted Lists

class Solution(object):
    def mergeTwoLists(self, list1, list2):
        cur1, cur2 = list1, list2

        cur = dummy = ListNode()

        while cur1 and cur2:
            if cur1.val < cur2.val:
                cur.next = cur1
                cur1 = cur1.next
            else:
                cur.next = cur2 
                cur2 = cur2.next
            cur = cur.next


        if cur1:
            cur.next = cur1
        else:
            cur.next = cur2


        return dummy.next