[LeetCode] Merge Two Sorted Lists

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.

注意写出鲁棒性的代码，时间复杂度O(n),空间复杂度O(1)

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