Implement check, create and iterate

src/aud/exam/prep/list/DoubleListItemProcessor.java

@@ -3,4 +3,10 @@ package aud.exam.prep.list;
 import aud.exam.prep.SequenceProcessor;
 
 abstract class DoubleListItemProcessor<T> implements SequenceProcessor<T, DoubleListItem<T>> {
+
+    @Override
+    public Iterable<T> iterate(DoubleListItem<T> list) {
+        return () ->
+            new ListItemIterator<>(list);
+    }
 }

src/aud/exam/prep/list/RecursiveDoubleListItemProcessor.java

@@ -3,6 +3,8 @@ package aud.exam.prep.list;
 import aud.exam.prep.Pair;
 
 import java.util.Comparator;
+import java.util.HashSet;
+import java.util.Iterator;
 
 public class RecursiveDoubleListItemProcessor<T> extends DoubleListItemProcessor<T> {
 
@@ -176,19 +178,58 @@ public class RecursiveDoubleListItemProcessor<T> extends DoubleListItemProcessor
 
     @Override
     public boolean check(DoubleListItem<T> list) {
-        // TODO: Your training
-        throw new RuntimeException("unimplemented");
+        return checkRec(list, new HashSet<>(), null);
+    }
+
+    private boolean checkRec(DoubleListItem<T> list, HashSet<DoubleListItem<T>> seen, DoubleListItem<T> prev) {
+        if (list == null) {
+            if (seen.isEmpty()) {
+                return true;
+            }
+
+            if (prev == null) {
+                return true;
+            }
+
+            return prev.next == null;
+        }
+
+        if (list.prev != prev) {
+            return false;
+        }
+
+        if (prev != null && prev.next != list) {
+            return false;
+        }
+
+        if (list.key == null) {
+            return false;
+        }
+
+        if (seen.contains(list)) {
+            return false;
+        }
+
+        seen.add(list);
+        return checkRec(list.next, seen, list);
     }
 
     @Override
     public DoubleListItem<T> create(Iterable<T> iterable) {
-        // TODO: Your training
-        throw new RuntimeException("unimplemented");
+        var iter = iterable.iterator();
+        return createRec(iter, null);
     }
 
-    @Override
-    public Iterable<T> iterate(DoubleListItem<T> list) {
-        // TODO: Your training
-        throw new RuntimeException("unimplemented");
+    private DoubleListItem<T> createRec(Iterator<T> iter, DoubleListItem<T> prev) {
+        if (!iter.hasNext()) {
+            return null;
+        }
+
+        var item = new DoubleListItem<T>();
+        item.key = iter.next();
+        item.next = createRec(iter, item);
+        item.prev = prev;
+
+        return item;
     }
 }