Leetcode: Sort List

Sort a linked list in O(n log n) time using constant space complexity.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *sortList(ListNode *head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        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;
    }
};