import java.io.*;
import java.util.*;

class Solution {
  public static List<List<String>> create_workflow_stages(List<List<String>> precursor_steps) {
    //
    Map<String, Set<String>> dep = new HashMap<>();

    for (List<String> pair : precursor_steps) {
        String key = pair.get(0);

        Set<String> dependency = dep.get(key);

        if (dependency == null) {
        dependency = new HashSet<>();
        dep.put(key, dependency);
        }

       dependency.add(pair.get(1));

    }

    List<List<String>> ret = new ArrayList<>();

    // Determine keys
    Set<String> visit = new HashSet<>();
    Set<String> data = dep.keySet();
    while(data.equals(visit)) {
      List<String> level = new ArrayList<>();

      for (String s : data) {
       if (visit.contains(s))
         continue;


      if (isSatisfied(visit, dep.get(s))) {
        level.add(s);
        visit.add(s);
      }
      }

      ret.add(level);
    }

    return ret;
  }

 static boolean isSatisfied(Set<String> visit, Set<String> deps) {
  for (String s : deps) {
   if (!visit.contains(s)) {
    return false;
   }
  }
   return true;
 }



  /* START TEST CASES
   * You can add test cases in the two lists below. Each test case
   * input/expected output should correspond to the same index in the
   * respective lists returned.
   */
  static List<List<List<String>>> testInputs = Arrays.asList(
    Arrays.asList(
      Arrays.asList("clean","build"),
      Arrays.asList("metadata","binary"),
      Arrays.asList("build","link"),
      Arrays.asList("link","binary"),
      Arrays.asList("clean","metadata"),
      Arrays.asList("build","resources")
    ),
    Arrays.asList(
      Arrays.asList("boil","serve"),
      Arrays.asList("chop", "boil"),
      Arrays.asList("stir", "boil"),
      Arrays.asList("set table", "serve")
    )
  );

  static List<List<List<String>>> testOutputs = Arrays.asList(
    Arrays.asList(
      Arrays.asList("clean"),
      Arrays.asList("build","metadata"),
      Arrays.asList("resources","link"),
      Arrays.asList("binary")
    ),
    Arrays.asList(
      Arrays.asList("chop","stir","set table"),
      Arrays.asList("boil"),
      Arrays.asList("serve")
    )
  );
  // END TEST CASES

  // DO NOT MODIFY BELOW THIS LINE
  public static boolean equalOutputs(List<List<String>> a, List<List<String>> b) {
    if (a.size() != b.size()) {
      return false;
    }
    for (int i = 0; i < a.size(); i++) {
      List<String> a1 = a.get(i);
      List<String> b1 = b.get(i);
      a1.sort(null);
      b1.sort(null);
      if (!a1.equals(b1)) {
        return false;
      }
    }
    return true;
  }

  public static void main(String[] args) {
    for (int i = 0; i < testInputs.size(); i++) {
      try {
        List<List<String>> result = create_workflow_stages(testInputs.get(i));

        if (equalOutputs(result, testOutputs.get(i))) {
          System.out.println("Pass");
        } else {
          System.out.println("Fail");
        }
      } catch (Exception e) {
        System.out.println("Fail");
        System.out.println(e);
      }
    }
  }
}