用c＋＋将两个升序链表合并成一个升序链表

#include<iostream>
using namespace std;

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
};

ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
    if(l1 == NULL) return l2;
    if(l2 == NULL) return l1;
    ListNode* head = NULL;
    if(l1->val < l2->val) {
        head = l1;
        head->next = mergeTwoLists(l1->next, l2);
    }
    else {
        head = l2;
        head->next = mergeTwoLists(l1, l2->next);
    }
    return head;
}

int main() {
    ListNode* l1 = new ListNode(1);
    l1->next = new ListNode(2);
    l1->next->next = new ListNode(4);
    ListNode* l2 = new ListNode(1);
    l2->next = new ListNode(3);
    l2->next->next = new ListNode(4);
    ListNode* l3 = mergeTwoLists(l1, l2);
    while(l3 != NULL) {
        cout << l3->val << " ";
        l3 = l3->next;
    }
    return 0;
}