题解 | 链表内指定区间反转

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 *   public ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @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
        ListNode pre = null, cur = head;
	    // 找到要翻转的开始位置
        for (int i = 0; i < m - 1; i++) {
            pre = cur;
            cur = cur.next;
        }
	  	// 如果n大于m，那么才需要翻转，并调用翻转前k位的方法
        if (n > m) {
            ListNode node = reverseK(cur, n - m + 1);
		  	// !!! 边界判断
            if (pre == null) return node;
            pre.next = node;
        }
        return head;
    }
    private ListNode reverseK(ListNode head, int k) {
        ListNode hair = null, cur = head, next;
        while (k-- > 0) {
            next = cur.next;
            cur.next = hair;
            hair = cur;
            cur = next;
        }
        head.next = cur;
        return hair;
    }
}