21. Merge Two Sorted Lists.py

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
        
        # Return the head of list1 or list2, if one is non-empty
        if not list1 or not list2:
            return list1 or list2
        
        # Recursively add sequential values to list1 if it contains smallest value
        # Vice versa for list2
        if list1.val < list2.val:
            list1.next = self.mergeTwoLists(list1.next, list2)
            return list1
        else:
            list2.next = self.mergeTwoLists(list2.next, list1)
            return list2