Merge Two Sorted Lists (LeetCode #21)

Challenge Statement

• You are given the heads of two sorted linked lists list1 and list2.
• Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists..
• Return the head of the merged linked list.
• This challenge corresponds to LeetCode #21.

Constraints

• The number of nodes in both lists is in the range [0, 50].
• -100 <= Node.val <= 100
• Both list1 and list2 are sorted in non-decreasing order.

Example 1:

Input: list1 = [1, 2, 4], list2 = [1, 3, 4]
Output: [1, 1, 2, 3, 4, 4]

Example 2:

Input: list1 = [], list2 = []
Output: []

Example 3:

Input: list1 = [], list2 = [0]
Output: [0]

Solution

Below is my solution and some test cases. This solution has a linear time complexity O(n) and a constant space complexity O(1), where n is the length of the input list.