21. Merge Two Sorted Lists

/**
 * 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) {
        
        ListNode start = null;
        
        if(l1 != null && l2 == null){
            return l1;
        }
        if(l1 == null && l2 != null){
            return l2;
        }
        if(l1 != null && l2 != null){
            if(l1.val < l2.val){
                start = l1;
                start.next = mergeTwoLists(l1.next, l2);
            }else{
                start = l2;
                start.next = mergeTwoLists(l1, l2.next);
            }
        }
        return start;
    }
}