题解 | #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
        if(head.next==null){
            return head;
        }
        if(m==n){
            return head;
        }
        ListNode start =head;
        int size=0;
        while(start!=null){
            start=start.next;
            size++;
        }
        start=head;
        ListNode end1 =head;
        for(int i=1;i<m-1;i++){
            start=start.next;
        }
       
        for(int i=1;i<n;i++){
            end1=end1.next;
        }
        ListNode start1=start.next;
        ListNode end2=end1.next;

        if(m==1){
            
            start1=head;
        }
            
        ListNode cur =start1.next;
        ListNode curNext=cur.next;
        ListNode h=start1;
        h.next=null;
        end1.next=null;
        while(cur!=null){
            curNext=cur.next;
            cur.next=h;
            h=cur;
            cur=curNext;
        }
        if(m==1){
            if(n==size){
                return end1;
            }else{
                start1.next=end2;
                return end1;
            }
        }
        
        if(n==size){
            
            start.next=end1;
            return head;
        }
        start.next=end1;
        start1.next=end2;

        return head;
    }
}