/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(not l1) return l2;
        if(not l2) return l1;
        if(l1->val > l2->val)
            swap(l1, l2);
        l1->next = mergeTwoLists(l1->next, l2);
        return l1;
    }
};