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(head==NULL || m==n) return head;                ListNode *p = head, *pp = NULL, *q = NULL;        --n;        while(--m && --n){ pp = p; p = p->next; }                ListNode *pfirst = pp, *first = p;            q = p->next;        while(n--){ pp = p; p = q; q = q->next; p->next = pp; }                if(pfirst == NULL) head = p;        else pfirst->next = p;        first->next = q;                return head;    }};