RoundTableKnights.java

/** https://codeforces.com/problemset/problem/71/C #implementation */
import java.util.ArrayList;
import java.util.HashSet;
import java.util.Scanner;

public class RoundTableKnights {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    ArrayList<Integer> goodMood = new ArrayList<>();
    for (int i = 1; i <= n; i++) {
      int v = sc.nextInt();
      if (v == 1) {
        goodMood.add(i);
      }
    }
    // corner case
    if (goodMood.size() < 3) {
      System.out.println("NO");
      return;
    }

    HashSet<Integer> set = new HashSet<>();
    goodMood.forEach(x -> set.add(x));
    int maxEdgeLen = n / 3;
    for (int edgeLen = 1; edgeLen <= maxEdgeLen; edgeLen++) {
      if (n % edgeLen != 0) continue;

      int edgeNum = n / edgeLen;
      for (int i = 1; i <= edgeLen; i++) {
        int cnt = 0;
        if (goodMood.size() < i) break;
        for (int point = goodMood.get(i - 1); set.contains(point); point += edgeLen) {
          cnt++;
        }
        if (cnt == edgeNum) {
          System.out.println("YES");
          return;
        }
      }
    }
    System.out.println("NO");
  }
}