25. K 个一组翻转链表

思路

• 先确定能翻转几组
• 每组内进行m到n的反转链表
• 对于反转m到n的问题，确定好循环次数，pre是不动的,循环内：
  • 先cur.next = ...
  • 再tmp.next = ...
  • 再pre.next = ...

class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        ListNode dummy = new ListNode(-1);
        dummy.next = head;
        //计算链表长度
        int size = 0;
        ListNode node = head;
        while (node != null) {
            size++;
            node = node.next;
        }
        int round = size / k;//轮数,表示一共进行round次k个为一组的翻转
        //开始翻转
        ListNode pre = dummy;
        ListNode cur = dummy.next;
        for (int i = 0; i < round; i++) {
            for (int j = 1; j < k; j++) {//以k为大小的区间内相邻节点挨个翻转，类似翻转m到n
                ListNode tmp = cur.next;
                cur.next = tmp.next;
                tmp.next = pre.next;
                pre.next = tmp;
            }
            //一组的翻转结束，往后挪
            pre = cur;
            cur = pre.next;
        }
        return dummy.next;
    }
}