LeetCode Merge Two Sorted Lists

Description

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.

The original problem is here.

The original code is here.

My Solution

I solve this problem in C++, as below:

/*
*Merge Two Sorted Lists
*Author: shuaijiang
*Email: zhaoshuaijiang8@gmail.com
*/
#include<iostream>
#include<math.h>
#include<vector>
#include<stdlib.h>
using namespace std;
/**
 * 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 && l2 != NULL)
            return l2;
        if(l1 != NULL && l2 == NULL)
            return l1;
        if(l1 == NULL && l2 == NULL)
            return NULL;
        
        ListNode *head, *list;
        if(l1->val < l2->val){
            head = l1;
            l1 = l1->next;    
        }
        else{
            head = l2;
            l2 = l2->next;
        }
        list = head;
        while(l1!=NULL && l2!=NULL){
            if(l1->val < l2->val){
                list->next = l1;
                list = list->next;
                l1 = l1->next;
            }
            else{
                list->next = l2;
                list = list->next;
                l2 = l2->next;
            }
        }
        if(l1 != NULL)
            list->next = l1;
        if(l2 != NULL)
            list->next = l2;
        
        return head;
        
    }
};

Note

To solve the problem, merge sort algorithm is need.