LeetCode:Merge Two Sorted Lists

Title:

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.

https://leetcode.com/problems/merge-two-sorted-lists/

非递归：

class Solution{
public:
    ListNode* mergeTwoLists(ListNode* l1,ListNode* l2){
        ListNode * head = new ListNode(0);
        ListNode* p1 = l1;
        ListNode* p2 = l2;
        ListNode* p = head;
        while (p1 != NULL && p2 != NULL){
            if (p1->val < p2->val){
                p->next = p1;
                p1 = p1->next;
            }else{
                p->next = p2;
                p2 = p2->next;
            }
            p = p->next;
        }
        if (p1 != NULL)
            p->next = p1;
        if (p2 != NULL)
            p->next = p2;
        ListNode* ret = head->next;
        delete head;
        return ret;
        
    }
};

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