【简单】合并两个有序链表

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
        ListNode *dummy = new ListNode(-1);
        ListNode *p = dummy, *p1 = list1, *p2 = list2;
        while (p1 != NULL && p2 != NULL)
        {
            if (p1->val > p2->val)
            {
                p->next = p2;
                p2 = p2->next;
            }
            else
            {
                p->next = p1;
                p1 = p1->next;
            }
            p = p->next;
        }
        if (p1 != NULL)
            p->next = p1;
        if (p2 != NULL)
            p->next = p2;
        return dummy->next;
    }
};