21. Merge Two Sorted Lists

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.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int val=0, ListNode next=null) {
 *         this.val = val;
 *         this.next = next;
 *     }
 * }
 */
public class Solution {
    public ListNode MergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null && l2 != null){
          return l2;
        }
        if(l1 != null && l2 == null){
          return l1;
        }
      
        var newnode = new ListNode(0);
        var currentnode = newnode;
          
        while(l1 != null && l2 != null){
          if(l1.val < l2.val){
            currentnode.next = l1;
            l1 = l1.next;
          }
          else{
            currentnode.next = l2;
            l2 = l2.next;
          }
          currentnode = currentnode.next;
        }
        
        if(l1 != null){
          currentnode.next = l1;
        }
        if(l2 != null){
          currentnode.next = l2;
        }
      
        return newnode.next;
    }
}