https://www.luogu.com.cn/blog/zyc-Kevin-Luogu-blog/bai-yuan-mai-bai-ji-wen-ti
第二种解法是 O(1)O(1)O(1) 还是 O(n2)O(n^2)O(n2) 还是 O(660)O(660)O(660)？
