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.

遍历一下，找到m,n，记录m的前驱和n的后继，对m到n这一段调用单链表反转

public class Solution {
 public ListNode reverseBetween(ListNode head, int m, int n)
	{
		int cnt=1;
		ListNode dummy=new ListNode(-1);
		dummy.next=head;
		ListNode prev=dummy;
		ListNode p=head;
		ListNode leftprev=prev;
		ListNode lefthead=p;
		ListNode righthead=p;
		ListNode rightnext=p.next;
		
		while(p!=null)
		{
			prev=p;
			p=p.next;
			cnt++;
			if(cnt==m)
			{
				leftprev=prev;
				lefthead=p;
			}
			if(cnt==n)
			{
				righthead=p;
				rightnext=p.next;
			}
		}
		
		righthead.next=null;
		leftprev.next=reverse(lefthead);
		lefthead.next=rightnext;
		
		return dummy.next;
		
	}
	
	
	private ListNode reverse(ListNode head)
	{
		if(head==null)
			return head;
		
		ListNode prev=head;
		ListNode p=head.next;
		ListNode pnext=null;
		while(p!=null)
		{
			pnext=p.next;
			p.next=prev;
			prev=p;
			p=pnext;
		}
		
		head.next=null;
		return prev;
	}
}