Rewrite RecursiveIterator solution to add support of disabled test

Author: andrei-punko

src/main/java/by/andd3dfx/iterators/RecursiveIterator.java

@@ -18,42 +18,56 @@
 public class RecursiveIterator<Object> implements Iterator<Object> {
 
     private final Deque<Iterator<Object>> stack = new ArrayDeque<>();
+    private Object nextElement;
 
     public RecursiveIterator(Iterator<Object> iterator) {
         stack.push(iterator);
     }
 
     @Override
     public boolean hasNext() {
-        if (stack.isEmpty()) {
-            return false;
-        }
-
-        Iterator<Object> currentIterator = stack.peek();
-        if (currentIterator.hasNext()) {
+        if (nextElement != null) {
             return true;
         }
-        stack.pop();
-        return hasNext();
+        nextElement = determineNextElement();
+        return nextElement != null;
     }
 
     @Override
     public Object next() {
-        if (stack.isEmpty()) {
+        var result = determineNextElement();
+        if (result == null) {
             throw new NoSuchElementException();
         }
+        return result;
+    }
 
-        Iterator<Object> currentIterator = stack.peek();
-        if (!currentIterator.hasNext()) {
-            stack.pop();
-            return next();
+    private Object determineNextElement() {
+        if (nextElement != null) {
+            var result = nextElement;
+            nextElement = null;
+            return result;
         }
 
-        Object object = currentIterator.next();
-        if (object instanceof String) {
-            return object;
+        if (stack.isEmpty()) {
+            return null;
+        }
+
+        while (!stack.isEmpty()) {
+            Iterator<Object> iterator = stack.peek();
+            if (iterator.hasNext()) {
+                var next = iterator.next();
+
+                if (next instanceof Iterator) {
+                    stack.push((Iterator<Object>) next);
+                    return determineNextElement();
+                } else {
+                    return next;
+                }
+            } else {
+                stack.pop();
+            }
         }
-        stack.push((Iterator<Object>) object);
-        return next();
+        return null;
     }
 }

src/test/java/by/andd3dfx/iterators/RecursiveIteratorTest.java

@@ -42,7 +42,6 @@ public void testNextHasNextForComplexStructure() {
         assertThat(result).isEqualTo(List.of("1", "3", "9", "4", "23", "31", "88", "22", "5", "37", "11"));
     }
 
-    @Ignore("TODO: change implemetation to support this case")
     @Test
     public void testNextForListOfEmptyLists() {
         List<Object> list = Arrays.asList(List.of().iterator(), List.of(List.of().iterator()).iterator());