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) {
if(head==NULL)
return NULL;
if(m==n)
return head;

ListNode* last=new ListNode(0);
last->next=head;

ListNode* p1=head;
head=last;
for(int i=1;i<m;i++)
{
p1=p1->next;
last=last->next;
}
ListNode* begin=p1;
ListNode* p2=p1->next;
ListNode* p3=NULL;
for(int i=m;i<n;i++)
{
p3=p2->next;
p2->next=p1;
p1=p2;
p2=p3;
}
last->next=p1;
begin->next=p2;
return head->next;
}
};