Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Solution:

Do the change on the original two lists.

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* head = new ListNode(0);
        ListNode* p1 = l1;
        ListNode* p2 = l2;
        ListNode* p = head;
        while (p1 && p2) {
            if (p1->val < p2->val) {
                p->next = p1;
                p1 = p1->next;
            } else {
                p->next = p2;
                p2 = p2->next;
            }
            p = p->next;
        }
        if (!p1) p->next = p2;
        else p->next = p1;
        return head->next;
        // Here maybe you should also consider freeing the head node.
    }
};