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(head==NULL)            return NULL;                    ListNode *p=head,*q,*pre,*next;        int k=0;                pre=NULL;        while(++k<m)        {            pre=p;            p=p->next;        }        q=pre;                ListNode *e=p;        pre=p;        p=p->next;        while(++k<=n)        {            next=p->next;            p->next=pre;            pre=p;            p=next;        }        e->next=p;        if(q)            q->next=pre;        else             head=pre;        return head;    }};