leetcode--Reverse Linked List II

[code]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.

[code]/**

* 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

if(m>=n)return head;

if(!head)return NULL;

int count=1;

ListNode * cur=head;

ListNode * start=NULL;

ListNode * headPrev=NULL;

while(count<m&&cur)

{

    headPrev=cur;

    cur=cur->next;count++;

}

start=cur;

while(count<n&&cur->next)

{

    ListNode *temp = cur->next;

    cur->next=temp->next;

    temp->next=start;

    start=temp;

    if(headPrev)headPrev->next=start;

    count++;

}

if(headPrev)return head;

else return start;

}

};