[leetcode]sort list

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
* Sort a linked list in O(n log n) time using constant space complexity.
*/
#include<iostream>
#include<fstream>
#include<vector>
#include<map>

using namespace std;

struct ListNode {
	int val;
	ListNode *next;
	ListNode(int x) : val(x), next(NULL) {}
};

class Solution {
public:
	ListNode *sortList(ListNode *head) {
		if (head == NULL || head->next == NULL)return head;
		map<int, vector<ListNode*>> mp;
		ListNode* pnode = head;
		while (pnode)
		{
			mp[pnode->val].push_back(pnode);
			pnode = pnode->next;
		}
		map<int, vector<ListNode*>>::iterator it = mp.begin();
		head = NULL;
		ListNode* cur = NULL;
		for (; it != mp.end(); it++)
		{
			vector<ListNode*> vec = (*it).second;
			for (int i = 0; i < vec.size(); i++)
			{
				if (head == NULL){
					head = vec[i];
					cur = vec[i];
				}
				else{
					cur->next = vec[i];
					cur = cur->next;
				}
			}
		}
		cur->next = NULL;
		return head;
	}
};

int main(){
	fstream fin("test.txt");
	struct ListNode* head(0);
	int val = 0;
	//string s1 = "asdf";
	//string s2(s1);
	//if (&s1 == &s2){
	//	int a = 1;
	//}
	while (fin >> val){
		if (NULL == head){
			head = new ListNode(val);
		} 
		else {
			ListNode *temp = head;
			while (temp->next != NULL)
				temp = temp->next;
			temp->next = new ListNode(val);
		}
	}
	Solution solution;
	ListNode *new_head = solution.sortList(head);
	return 0;

}

主函數裏關於鏈表的建立，使用了new從自由存儲區（堆）中分配了內存，鏈表一直使用到程序結束，有必要顯示的用delete進行內存釋放嗎？

關於鏈表的建立可以不使用new或malloc進行分配嗎，有其他的方法嗎？