25. K 个一组翻转链表

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode reverseKGroup(ListNode head, int k) {
        Deque<ListNode> stack = new ArrayDeque<ListNode>();
        ListNode dummy = new ListNode(0);
        ListNode p = dummy;
        while (true) {
            int count = 0;
            ListNode tmp = head;
            while (tmp != null && count < k) {
                stack.add(tmp);
                tmp = tmp.next;
                count++;
            }
            if (count != k) {
                p.next = head;
                break;
            }
            while (!stack.isEmpty()){
                p.next = stack.pollLast();
                p = p.next;
            }
            p.next = tmp;
            head = tmp;
        }
        return dummy.next;
    }
}
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用栈的方法，空间复杂度应该是O(k)，而不是常数的额外空间