#include "s0025_reverse_nodes_in_k-group.hpp"

// 翻转一个子链表，并且返回新的头与尾
pair<ListNode*, ListNode*> S0025::myReverse(ListNode* head, ListNode* tail) {
  ListNode* prev = tail->next;
  ListNode* p = head;
  while (prev != tail) {
    ListNode* nex = p->next;
    p->next = prev;
    prev = p;
    p = nex;
  }
  return {tail, head};
}

ListNode* S0025::reverseKGroup(ListNode* head, int k) {
  ListNode* hair = new ListNode(0);
  hair->next = head;
  ListNode* pre = hair;

  while (head) {
    ListNode* tail = pre;
    // 查看剩余部分长度是否大于等于 k
    for (int i = 0; i < k; ++i) {
      tail = tail->next;
      if (!tail) {
        return hair->next;
      }
    }
    ListNode* nex = tail->next;
    // 这里是 C++17 的写法，也可以写成
    // pair<ListNode*, ListNode*> result = myReverse(head, tail);
    // head = result.first;
    // tail = result.second;
    tie(head, tail) = myReverse(head, tail);
    // 把子链表重新接回原链表
    pre->next = head;
    tail->next = nex;
    pre = tail;
    head = tail->next;
  }

  return hair->next;
}