21. 合并两个有序链表

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {

        ListNode *pHead = new ListNode(-1);
        ListNode *p = pHead;
        while (l1 != NULL && l2 != NULL)
        {
            if (l1->val <= l2->val) {
                p->next = l1;
                p = l1;
                l1 = l1->next;
            }
            else {
                p->next = l2;
                p = l2;
                l2 = l2->next;
            }
        }

        p->next = (l1 == NULL ? l2 : l1);

        return pHead->next;

    }
};