Merge Two Sorted Lists

From LeetCode

problem description / no solution provided

Solution in Python3

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def mergeTwoLists(
            self, 
            l1: ListNode, 
            l2: ListNode
        ) -> ListNode:
        if not l2:
            return l1
        if not l1:
            return l2
        if l1.val < l2.val:
            l1.next = self.mergeTwoLists(l1.next, l2)
            return l1
        else:
            l2.next = self.mergeTwoLists(l1, l2.next)
            return l2

I am lucky to get

Runtime: 44 ms, faster than 87.36% of Python3 online submissions for Merge Two Sorted Lists.

Time Complexity

\(O(\max(l_1, l_2))\), where \(l_1\) and \(l_2\) are the length of l1 and l2, respectively.

Variants

Iteration

class Solution:
    def mergeTwoLists(
            self, 
            l1: ListNode, 
            l2: ListNode
        ) -> ListNode:
        current = head = ListNode(0)
        while l1 and l2:
            if l1.val < l2.val:
                current.next = l1
                l1 = l1.next
                current = current.next               
            else:
                current.next = l2
                l2 = l2.next
                current = current.next
        if l1:
            current.next = l1
        elif l2:
            current.next = l2
        return head.next

I got:

Runtime: 44 ms, faster than 87.36% of Python3 online submissions for Merge Two Sorted Lists.

Not a surprise since time complexity is the same as the recursive solution above. Memory usage could be less, though.