/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if(l1 == null && l2 != null)return l2;
        else if(l1 != null && l2 == null)return l1;
        else if(l1 == null && l2 == null)return l1;
        if(l1.val > l2.val){
            ListNode res = mergeTwoLists(l2,l1);
            return res;
        }
        ListNode res = l1, p = l1, q = l2;
        while(p!=null && q!=null){
            l2 = q;
            if(p.val <= q.val && p.next!=null && p.next.val >= q.val){
                q = q.next;
                l2.next = p.next;
                p.next = l2;
            }else if(p.next!=null && p.next.val <= q.val)p = p.next;
            else if(p.next == null){p.next = q;return res;}
        }
        return res;
    }
}