[Leetcode]_21 Merge Two Sorted Lists

/**
*  Index: 21
*  Title: Merge Two Sorted Lists
*  Author: ltree98
**/

第三方介入

/**
* 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* l3 = new ListNode(0);
ListNode* head = l3;

while(l1 && l2) {
if(l1->val <= l2->val)  {
ListNode* temp = new ListNode(l1->val);
l3->next = temp;
l1 = l1->next;
}
else    {
ListNode* temp = new ListNode(l2->val);
l3->next = temp;
l2 = l2->next;
}
l3 = l3->next;
}

if(l1)
l3->next = l1;
else
l3->next = l2;

return head->next;
}
};

递归

/**
* 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;

if(l1->val <= l2->val)  {
l1->next = mergeTwoLists(l1->next, l2);
return l1;
}
else    {
l2->next = mergeTwoLists(l1, l2->next);
return l2;
}
}
};