"""

Merge two sorted linked lists and return it as a sorted list. The list should be made by splicing together the nodes of the first two lists.

 

Example 1:


Input: l1 = [1,2,4], l2 = [1,3,4]
Output: [1,1,2,3,4,4]
Example 2:

Input: l1 = [], l2 = []
Output: []
Example 3:

Input: l1 = [], l2 = [0]
Output: [0]
 

Constraints:

The number of nodes in both lists is in the range [0, 50].
-100 <= Node.val <= 100
Both l1 and l2 are sorted in non-decreasing order.

"""

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
      # maintain an unchanging reference to node ahead of the return node.
        prehead = ListNode(-1)

        prev = prehead
        while l1 and l2:
            if l1.val <= l2.val:
                prev.next = l1
                l1 = l1.next
            else:
                prev.next = l2
                l2 = l2.next            
            prev = prev.next

        # At least one of l1 and l2 can still have nodes at this point, so connect
        # the non-null list to the end of the merged list.
        prev.next = l1 if l1 is not None else l2

        return prehead.next
      
        # Time Complexity:O(n)
        # Space Complexity: O(n) to store the new linked list
        
