[Leetcode] Merge Two Sorted Lists

//recursion

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (!l1)
            return l2;
        if (!l2)
            return l1;
            
        if (l1->val < l2->val)
        {
            l1->next = mergeTwoLists(l1->next, l2);
            return l1;
        }
        else
        {
            l2->next = mergeTwoLists(l1, l2->next);
            return l2;
        }
    }
};

//non recurision
class Solution {
public:
    ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
        ListNode* res = NULL;
        ListNode** cur = &res;
        
        while (l1 && l2)
        {
            if (l1->val < l2->val)
            {
                *cur = l1;
                cur = &(l1->next);
                l1 = l1->next;
            }
            else
            {
                *cur = l2;
                cur = &(l2->next);
                l2 = l2->next;
            }            
        }
        
        if (l1)
            *cur = l1;
        else
            *cur = l2;
            
        return res;
    }
};