/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(l1 == nullptr) return l2;
        if(l2 == nullptr) return l1;
        if(l1 -> val <= l2 -> val) {
            l1 -> next = mergeTwoLists(l1 -> next, l2);
            return l1;
        }
        l2 -> next = mergeTwoLists(l2 -> next, l1);
        return l2;
    }
};