[LeetCode]Merge Two Sorted Lists

题目描述:(链接)

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.

解题思路：

/**
 * 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) return l2;
        if (!l2) return l1;
        
        ListNode dummy(-1);
        ListNode *p = &dummy;
        while (l1 != nullptr && l2 != nullptr) {
            if (l1->val < l2->val) {
                p->next = l1;
                l1 = l1->next;
            } else {
                p->next = l2;
                l2 = l2->next;
            }
            p = p->next;
        }
        
        if (!l1) {
            p->next = l2;
        } else {
            p->next = l1;
        }
        
        return dummy.next;
    }
};