후보키

package programmers;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class 후보키 {

   static List<Set<Character>> candidateKeys;
   static String[][] table;

   public static int solution(String[][] relation) {
      String columns = init(relation);

      for (int i = 1; i < table[0].length + 1; i++) {
         go(columns, new HashSet<>(), i);
      }

      return candidateKeys.size();
   }

   private static void go(String columns, Set<Character> out, int r) {
      if (r == 0) {
         if (isUnique(out) && isMinimal(out)) {
            candidateKeys.add(out);
         }
         return;
      }

      for (int i = 0; i < columns.length(); i++) {
         Set<Character> newOut = new HashSet<>(out);
         newOut.add(columns.charAt(i));
         go(columns.substring(i + 1), newOut, r - 1);
      }
   }

   private static boolean isUnique(Set<Character> key) {
      Set<String> set = new HashSet<>();
      for (String[] row : table) {
         StringBuilder sb = new StringBuilder();
         for (char col : key) {
            sb.append(row[col - '0']);
         }
         if (set.contains(sb.toString())) {
            return false;
         }
         set.add(sb.toString());
      }

      return true;
   }

   private static boolean isMinimal(Set<Character> key) {
      for (Set<Character> candidateKey : candidateKeys) {
         if (key.containsAll(candidateKey)) {
            return false;
         }
      }

      return true;
   }

   private static String init(String[][] relation) {
      table = relation;
      candidateKeys = new ArrayList<>();

      StringBuilder sb = new StringBuilder();
      for (int i = 0; i < table[0].length; i++) {
         sb.append(i);
      }

      return sb.toString();
   }

   public static void main(String[] args) {
      System.out.println(solution(new String[][] {
         {"100", "ryan", "music", "2"}, {"200", "apeach", "math", "2"},
         {"300", "tube", "computer", "3"}, {"400", "con", "computer", "4"},
         {"500", "muzi", "music", "3"}, {"600", "apeach", "music", "2"}}));
   }
}