/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {

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

        ListNode rootNode;
        if(l1.val < l2.val) {
            rootNode = l1;
            l1 = l1.next;
        } else {
            rootNode = l2;
            l2 = l2.next;
        }

        ListNode toReturn = rootNode;

        while (l1 != null && l2 != null) {
            if(l1.val < l2.val) {
                rootNode.next = l1;
                l1 = l1.next;
            } else {
                rootNode.next = l2;
                l2 = l2.next;
            }
            rootNode = rootNode.next;
        }
        if(l1 != null) {
            rootNode.next = l1;
        } else if (l2 != null) {
            rootNode.next = l2;
        }
        return toReturn;
    }
}