BM2 链表内指定区间反转

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param head ListNode类 
     * @param m int整型 
     * @param n int整型 
     * @return ListNode类
     */
    public ListNode reverseBetween (ListNode head, int m, int n) {
        // write code here
        //1.首先设置一个虚拟的头节点
        ListNode dummyNode = new ListNode(-1);
        dummyNode.next = head;
        
        //2.指向相应的标志位
        ListNode left = dummyNode;
        for(int i = 0;i < m-1;i++)
        {
            left = left.next;
        }
        ListNode first = left.next;
        ListNode last = first;
        for(int i = 0;i < n-m;i++)
        {
            last = last.next;
        }
        ListNode right = last.next;
        
        //3.断开相应的链接，形成真正的子链
        left.next = null;
        last.next = null;
        
        //4.调用反转的函数进行反转
        reverseList(first);
        
        //5.最后重新进行链接
        left.next = last;
        first.next = right;
        
        //6.返回头节点
        return dummyNode.next;
    }
    
    public ListNode reverseList(ListNode head)
    {
        ListNode pre = head;
        ListNode cur = head.next;
        ListNode next = null;
        while(cur != null)
        {
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
}