_92.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.ListNode;

public class _92 {

    public static class Solution1 {
        /**
         * credit: https://discuss.leetcode.com/topic/8976/simple-java-solution-with-clear-explanation
         */
        public ListNode reverseBetween(ListNode head, int m, int n) {
            // use four nodes, pre, start, then, dummy
            // just reverse the nodes along the way
            ListNode dummy = new ListNode(-1);
            dummy.next = head;
            ListNode pre = dummy;
            for (int i = 0; i < m - 1; i++) {
                pre = pre.next;
            }

            ListNode start = pre.next;// start is the node prior to reversing, in the given example,
            // start is node with value 1
            ListNode then = start.next;// then is the node that we'll start to reverse, in the given
            // example, it's 2

            for (int i = 0; i < n - m; i++) {
                // pay special attention to this for loop, it's assigning then.next to start.next, it
                // didn't initialize a new node
                // this does exactly what I desired to do, but I just didn't figure out how to implement
                // it, thumbs up to the OP!
                start.next = then.next;
                then.next = pre.next;
                pre.next = then;
                then = start.next;
            }

            return dummy.next;
        }
    }
}