Solution.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode reverseEvenLengthGroups(ListNode head) {
        int n = 0;
        for (ListNode t = head; t != null; t = t.next) {
            ++n;
        }
        ListNode dummy = new ListNode(0, head);
        ListNode prev = dummy;
        int l = 1;
        for (; (1 + l) * l / 2 <= n && prev != null; ++l) {
            if (l % 2 == 0) {
                ListNode node = prev.next;
                prev.next = reverse(node, l);
            }
            for (int i = 0; i < l && prev != null; ++i) {
                prev = prev.next;
            }
        }
        int left = n - l * (l - 1) / 2;
        if (left > 0 && left % 2 == 0) {
            ListNode node = prev.next;
            prev.next = reverse(node, left);
        }
        return dummy.next;
    }

    private ListNode reverse(ListNode head, int l) {
        ListNode prev = null;
        ListNode cur = head;
        ListNode tail = cur;
        int i = 0;
        while (cur != null && i < l) {
            ListNode t = cur.next;
            cur.next = prev;
            prev = cur;
            cur = t;
            ++i;
        }
        tail.next = cur;
        return prev;
    }
}