(21)Merge two sorted linked list - merge sort

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        
        ListNode* result = NULL;
        
        if( l1 == NULL and l2 == NULL)
            return NULL;
        
        if( l1 == NULL)
            return l2;
        
        if( l2 == NULL)
            return l1;
        
        if( l1 != NULL and l2 != NULL)
        {
            if( l1->val < l2->val) {
                result = new ListNode(l1->val);
                result->next = mergeTwoLists(l1->next , l2);
            }
            else {
                result = new ListNode(l2->val);
                result->next = mergeTwoLists(l1, l2->next);
            }
        }
        return result;        
    }
};