Merge Two Sorted Lists

The problem description can be found in LeetCode.

Approach

It is very easy to think on a recursive approach here. In fact, we should take:

• list1[0] + merge(list1[1:], list2) if list1[0] < list2[0].
• list2[0] + merge(list1, list2[1:]) if list1[0] >= list2[0].

Solution

from typing import Optional


class Solution:
    def mergeTwoLists(
        self, list1: Optional[ListNode], list2: Optional[ListNode]
    ) -> Optional[ListNode]:
        if not list1 and not list2:
            return None
        if not list1:
            return list2
        if not list2:
            return list1

        if list1.val < list2.val:
            list1.next = self.mergeTwoLists(list1.next, list2)
            return list1
        else:
            list2.next = self.mergeTwoLists(list1, list2.next)
            return list2

Complexity

• Time: $O(n+m)$
• Space: $O(n+m)$

Where $n$ and $m$ are the sizes of the list1 and list2 lists, respectively.