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.

Merge Two Sorted Lists.cpp

/**
 * 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 == NULL and l2 == NULL) {
            return NULL;
        } else if(l1 == NULL) {
            return l2;
        } else if(l2 == NULL) {
            return l1;
        }
        
        ListNode tmpHead(-1);
        ListNode* cur = &tmpHead;
        
        while(l1 != NULL and l2 != NULL){
            if(l1->val > l2->val) {
                cur->next = l2;
                l2 = l2->next;
            } else {
                cur->next = l1;
                l1 = l1->next;
            }
            
            cur=cur->next;
        }
        
        
        if(l1 != NULL) {
            cur->next = l1;
        } else {
            cur->next = l2;
        }
        
        return tmpHead.next;

    }
};