LeetCode_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.

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        assert(m >= 1 && n >= 1 && m <= n);
        if(head == NULL) return head ;
        
        ListNode * headm,  *p, *q,*preP,*endm;
        
        //find the mth node
        int i = 1;
        p = head ;
        preP = NULL;
        while(i<m &&p){
            preP = p;
            p= p->next;
            i++;
        }
        
        //reverse Between m and n
        headm = NULL;
        endm = p;
        while(i<=n && p)
        {
          q = p;
          p = p->next;
          q->next = headm;
          headm = q;       
          i++;
        }
        
        if(preP == NULL)
            head = headm;
        else
            preP->next = headm ;
            
            
        if(endm)
             endm->next = p;

        return head;    
    }
};