# 21. Merge Two Sorted Lists 混合插入有序链表

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.

Example:

``````Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4
``````

Difficulty: Easy

### 解法1

``````/**
* 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;
if (l1->val<l2->val)
{
}
else
{
}
}
};
``````

### 解法2

``````/**
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode* dummy = new ListNode(-1), *cur = dummy;
while(l1&&l2)
{
if (l1->val < l2->val){
cur->next = l1;
l1 = l1->next;
}
else {
cur->next = l2;
l2 = l2->next;
}
cur = cur->next;
}
cur->next = l1?l1:l2;
//要记得释放没用的节点
ListNode* res = dummy->next;
delete dummy;
dummy = NULL;
return res;
}
};
``````