saddle-points: update API, list -> set

Order of identified saddle points is not important.

exercises/saddle-points/src/example/java/Matrix.java

-import java.util.ArrayList;
 import java.util.Collections;
+import java.util.HashSet;
 import java.util.List;
+import java.util.Set;
 
 final class Matrix {
 
     this.values = values;
   }
 
-  List<MatrixCoordinate> getSaddlePoints() {
-    final List<MatrixCoordinate> result = new ArrayList<>();
+  Set<MatrixCoordinate> getSaddlePoints() {
+    final Set<MatrixCoordinate> result = new HashSet<>();
 
     if (values.isEmpty()) {
       return result;
     return values.stream()
         .map(row -> row.get(column))
         .min(Integer::compareTo)
-        .get();
+        .orElseThrow(() -> new IllegalArgumentException("Column cannot be empty"));
   }
 
 }

exercises/saddle-points/src/test/java/MatrixTest.java

 import org.junit.Ignore;
 import org.junit.Test;
 
-import java.util.ArrayList;
-import java.util.Arrays;
-import java.util.Collections;
-import java.util.List;
+import java.util.*;
 
 import static org.junit.Assert.assertEquals;
 
             Arrays.asList(6, 6, 7)
         ));
 
-        List<MatrixCoordinate> expectedSaddlePoints = Collections.singletonList(new MatrixCoordinate(1, 0));
+        Set<MatrixCoordinate> expectedSaddlePoints = Collections.singleton(new MatrixCoordinate(1, 0));
 
         assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
     }
     public void testCanIdentifyThatEmptyMatrixHasNoSaddlePoints() {
         Matrix matrix = new Matrix(new ArrayList<>());
 
-        List<MatrixCoordinate> expectedSaddlePoints = new ArrayList<>();
+        Set<MatrixCoordinate> expectedSaddlePoints = Collections.emptySet();
 
         assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
     }
             Arrays.asList(2, 3, 1)
         ));
 
-        List<MatrixCoordinate> expectedSaddlePoints = new ArrayList<>();
+        Set<MatrixCoordinate> expectedSaddlePoints = Collections.emptySet();
 
         assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
     }
             Arrays.asList(1, 5, 4)
         ));
 
-        List<MatrixCoordinate> expectedSaddlePoints = Arrays.asList(
+        Set<MatrixCoordinate> expectedSaddlePoints = new HashSet<>(Arrays.asList(
             new MatrixCoordinate(0, 1),
             new MatrixCoordinate(1, 1),
             new MatrixCoordinate(2, 1)
-        );
+        ));
 
         assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
     }
             Arrays.asList(3, 2, 5)
         ));
 
-        List<MatrixCoordinate> expectedSaddlePoints = Collections.singletonList(new MatrixCoordinate(2, 2));
+        Set<MatrixCoordinate> expectedSaddlePoints = Collections.singleton(new MatrixCoordinate(2, 2));
 
         assertEquals(expectedSaddlePoints, matrix.getSaddlePoints());
     }