/*
 * @lc app=leetcode id=25 lang=java
 *
 * [25] Reverse Nodes in k-Group
 */

// @lc code=start
/**
 * 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 reverseKGroup(ListNode head, int k) {
        ListNode dummy = new ListNode(0), prev, frm, to;
        dummy.next = head;
        prev = dummy;
        while(prev.next != null) {
            frm = to = prev.next;
            for(int i = 0; i<k-1; i++) {
                if(to!=null) {
                    to = to.next;
                }
            }
            if(to == null) {
                // reach the end
                break;
            }
            // reverse nodes in [frm, to]
            ListNode cur = frm.next;
            for(int i = 0; i<k-1; i++) {
                frm.next = cur.next;
                cur.next = prev.next;
                prev.next = cur;
                cur = frm.next;
            }
            prev = frm;
        }
        return dummy.next;
    }
}
// @lc code=end