class Solution {public:    ListNode *reverseBetween(ListNode *head, int m, int n) {        if(m == n) return head;        ListNode *pPre = NULL;        ListNode *p1 = head;        ListNode *p2 = head->next;        ListNode *pPost = NULL;    if(p2) pPost = p2->next;                //    Find m first        int om = m;        while(--om)        {            if(!pPre) pPre = head;                    else pPre = pPre->next;            p1 = p2;            if(p2) p2 = p2->next;            if(pPost) pPost = pPost -> next;        }                ListNode *pPreM = pPre;        ListNode *pM = p1;        //    Reverse it        int cnt = n - m;        while(cnt --)        {            p1->next = pPre;                        p2->next = p1;            pPre = p1;            p1 = p2;            p2 = pPost;            if(pPost) pPost = pPost->next;        }                if(pPreM)    pPreM->next = p1;        else head = p1;        if(pM) pM->next = p2;        return head;    }};