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) {
        ListNode currNode = head;
        ListNode preNode = null;
        ListNode nextNode = null;
        ListNode firstNode = head;
        ListNode finalNode = null;
        ListNode currfirtNode = null;
//      1 -> 2 <- 3  4 -> 5 -> 6
        int i = 0;
        if (head.next == null) {
            return head;
        }
        while (currNode != null) {
            i++;
            nextNode = currNode.next;
            if (i < m) {
                firstNode = currNode;
            }
            if (i == m) {
                currfirtNode = currNode;
            }
            if ((n < i && finalNode == null) || (n == i && nextNode == null)) {
                finalNode = currNode;
            }
            if (i >= m && i <= n) {
                currNode.next = preNode;
                preNode = currNode;
            }
            currNode = nextNode;
        }
        if (currfirtNode != firstNode) {
            firstNode.next = preNode;
        } else {
            if (currfirtNode != finalNode && finalNode != preNode ) {
                currfirtNode.next = finalNode;
            }
            return preNode;
        }
//        1 -> 2
        if (currfirtNode != finalNode && finalNode != preNode ) {
            currfirtNode.next = finalNode;
        }
        return head;
    }
}