21. Merge Two Sorted Lists

class Solution:
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if not l1 or not l2:
            return l1 or l2

        res = ListNode(0)
        head = res

        temp1 = l1
        temp2 = l2
        while temp1 and temp2:
            if temp1.val <= temp2.val:
                head.next = temp1
                temp1 = temp1.next
            else:
                head.next = temp2
                temp2 = temp2.next

            head = head.next

        remain = temp1 or temp2
        head.next = remain

        return res.next