Reverse Linked List II

/*

Reverse Linked List II

Reverse a linked list from position m to n. Do it in-place and in one-pass.
For example:
Given 1->2->3->4->5->NULL, m = 2 and n = 4,
return 1->4->3->2->5->NULL.
Note:
Given m, n satisfy the following condition:
1 ? m ? n ? length of list.

*/
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
ListNode *reverseBetween(ListNode *head, int m, int n){
 if(m>n) return NULL;
 if(m==n) return head;
 if(m<1) return NULL;
 ListNode *prep = head;
 int i = 0;
 while(i<(m-1)){
  prep = prep->next;
  i++;
 }
 stack<int> s;
 ListNode *curr;
 curr = prep;
 s.push(curr->val);
 while(i<(n-1) && curr!=NULL){
  curr = curr->next;
  if(curr!=NULL) s.push(curr->val);
  i++;
 }
 while(!s.empty()){
  prep->val = s.top();
  prep = prep->next;
  s.pop();
 }
 return head;
}
    };