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
        if (head == null || n <= 1 || m == n) {
            return head;
        }

        ListNode pre = head;
        ListNode end = head;
        ListNode mid = head;
        ListNode cur = head;
        int curPosition = 1;

        if (m == 1) {
            return reverseNodesInBeginning(pre, end, mid, cur, m, n, curPosition);
        } else {
            return reverseNodesInMiddle(head, pre, end, mid, cur, m, n, curPosition);
        }
    }

    public static ListNode reverseNodesInBeginning(ListNode pre
            , ListNode end
            , ListNode mid
            , ListNode cur
            , int m
            , int n
            , int curPosition) {
        pre = cur.next;
        while (curPosition <= n) {
            if (curPosition < m) {
                pre = cur;
                cur = cur.next;
                curPosition++;
                continue;
            }
            if (curPosition == m) {
                end = cur;
                mid = cur;
                cur = cur.next;
                curPosition++;
                continue;
            }
            end.next = cur.next;
            cur.next = mid;
            mid = cur;
            cur = end.next;
            curPosition++;
        }
        return mid;
    }

    public static ListNode reverseNodesInMiddle(ListNode head
            , ListNode pre
            , ListNode end
            , ListNode mid
            , ListNode cur
            , int m
            , int n
            , int curPosition) {
        while (curPosition <= n) {
            if (curPosition < m) {
                pre = cur;
                cur = cur.next;
                curPosition++;
                continue;
            }
            if (curPosition == m) {
                end = cur;
                mid = cur;
                cur = cur.next;
                curPosition++;
                continue;
            }
            end.next = cur.next;
            cur.next = mid;
            mid = cur;
            cur = end.next;
            curPosition++;
        }
        pre.next = mid;
        return head;
    }

}