#include #include 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 *list1, ListNode *list2) { if (list1 == NULL) return list2; if (list2 == NULL) return list1; if (list1->val <= list2->val) { list1->next = mergeTwoLists(list1->next, list2); return list1; } else { list2->next = mergeTwoLists(list1, list2->next); return list2; } } }; int main() {}