21. Merge Two Sorted Lists

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

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

# Definition for singly-linked list.
# class ListNode
#     attr_accessor :val, :next
#     def initialize(val = 0, _next = nil)
#         @val = val
#         @next = _next
#     end
# end
# @param {ListNode} l1
# @param {ListNode} l2
# @return {ListNode}
def merge_two_lists(l1, l2)  
    new_node = current_node = ListNode.new(0)
  
    while l1 || l2
      if l1 && l2
        if l1.val <= l2.val
          current_node.next = l1
          l1 = l1.next
          current_node = current_node.next
        else
          current_node.next = l2
          l2 = l2.next
          current_node = current_node.next
        end
      elsif l1 && l2.nil?
          current_node.next = l1
          return new_node.next
      elsif l2 && l1.nil?
          current_node.next = l2
          return new_node.next
      end
    end
  
    return new_node.next
  
end