/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */

public class Solution {

    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {

        if (l1 == null) {
            return l2;
        }

        if (l2 == null) {
            return l1;
        }

        if (l1.val < l2.val) {

            l1.next = mergeTwoLists(l1.next, l2);
            return l1;

        } else {

            l2.next = mergeTwoLists(l1, l2.next);
            return l2;

        }

    }

}

Create : 21 Merge Two Sorted Lists
Create : 21 Merge Two Sorted Lists
