【leetcode】92. 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) {
        int d=n-m;
        if(head==NULL||head->next==NULL||d<=0)
            return head;
       
        ListNode* p=head,*bfp=p;
        ListNode* q=head;
        for(int i=0;i<m-1;i++){
            bfp=p;
            p=p->next;
        }
        ListNode* revhead=p;
        ListNode* bf=p,*bh=NULL;
        p=p->next;
        d--;
        while(p->next!=NULL&&d!=0){
            d--;
            bh=p->next;
            p->next=bf;
            bf=p;
            p=bh;
            
        }
        bh=p->next;
        p->next=bf;
        bfp->next=p;
        revhead->next=bh;
        if(m!=1)
            return head;
        else return p;
    }
};