LeetCode：合并两个有序链表

C++示例：

/**
 * 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* result = new ListNode(0);
        ListNode* p = result;
        while (l1 != NULL && l2 != NULL) {
            if (l1->val < l2->val) {
                p->next = l1;
                l1 = l1->next;
            } else {
                p->next = l2;
                l2 = l2->next;
            }
            p = p->next;
        }
        p->next = (l1 == NULL) ? l2 : l1;
        return result->next;
    }
};