15 Dec 21. Merge Two Sorted Lists
You are given the heads of two sorted linked lists
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.
Input: list1 = [1,2,4], list2 = [1,3,4] Output: [1,1,2,3,4,4]
Input: list1 = , list2 =  Output: 
Input: list1 = , list2 =  Output: