https://leetcode.cn/problems/merge-two-sorted-lists/

/**
 * 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 *head1, ListNode *head2)
        {
            ListNode *head3 = new ListNode(-1);
            ListNode *p = head3;

            while(head1 != nullptr && head2 != nullptr)
            {
                if(head1->val < head2->val)
                {
                    p->next = head1;
                    head1 = head1->next;
                }

                else
                {
                    p->next = head2;
                    head2 = head2->next;
                }

                p = p->next;
            }

            if(head1 != nullptr)
            {
                p->next = head1;
            }

            else
            {
                p->next = head2;
            }

            return head3->next;
        }
};