Leetcode 21 - 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.

1 - 简单的链表操作
2 - 应该注意到，题目要求新的链表应该由两个旧链表接合而成。因此，我们应该仅仅修改指针，而不应该去生成一个新链表。

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