NestedIterator: adjust javadoc, adjust solution & tests a bit; add YT link

Author: andrei-punko

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

@@ -14,7 +14,7 @@
  * or other lists. Implement an iterator to flatten it.
  *
  * Implement the NestedIterator class:
- *     NestedIterator(List<NestedInteger> nestedList) Initializes the iterator with the nested list nestedList.
+ *     NestedIterator(List&lt;NestedInteger&gt; nestedList) Initializes the iterator with the nested list nestedList.
  *     int next() Returns the next integer in the nested list.
  *     boolean hasNext() Returns true if there are still some integers in the nested list and false otherwise.
  *
@@ -29,15 +29,17 @@
  * If res matches the expected flattened list, then your code will be judged as correct.
  *
  * Example 1:
- * Input: nestedList = [[1,1],2,[1,1]]
- * Output: [1,1,2,1,1]
- * Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].
+ *   Input: nestedList = [[1,1],2,[1,1]]
+ *   Output: [1,1,2,1,1]
+ *   Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1].
  *
  * Example 2:
- * Input: nestedList = [1,[4,[6]]]
- * Output: [1,4,6]
- * Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].
+ *   Input: nestedList = [1,[4,[6]]]
+ *   Output: [1,4,6]
+ *   Explanation: By calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,4,6].
  * </pre>
+ *
+ * @see <a href="https://youtu.be/RiJ7Rc98vkI">Video solution</a>
  */
 public class NestedIterator implements Iterator<Integer> {
 
@@ -76,11 +78,11 @@ private Integer determineNextElement() {
         while (!stack.isEmpty()) {
             Iterator<INestedInteger> iterator = stack.peek();
             if (iterator.hasNext()) {
-                var next = iterator.next();
+                INestedInteger next = iterator.next();
 
-                if (next.isInteger()) {
+                if (next.isInteger()) {     // is integer
                     return next.getInteger();
-                } else {
+                } else {                    // is list
                     stack.push(next.getList().iterator());
                     return determineNextElement();
                 }
@@ -91,6 +93,9 @@ private Integer determineNextElement() {
         return null;
     }
 
+    /**
+     * This is the interface that allows for creating nested lists. You should not implement it, or speculate about its implementation
+     */
     public interface INestedInteger {
 
         // @return true if this NestedInteger holds a single integer, rather than a nested list.

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

@@ -13,6 +13,9 @@ public class NestedIteratorTest {
 
     @Test
     public void testNextAndHasNext() {
+        check(List.of(new NestedInteger(2)), List.of(2));
+        check(List.of(new NestedIntegerList(new NestedIntegerList())), List.of());
+
         check(List.of(
                 new NestedIntegerList(1, 1),
                 new NestedInteger(2),
@@ -72,7 +75,7 @@ public Integer getInteger() {
 
         @Override
         public List<INestedInteger> getList() {
-            throw new IllegalStateException();
+            throw new IllegalStateException("Could not call getList on Integer!");
         }
     }
 
@@ -102,7 +105,7 @@ public boolean isInteger() {
 
         @Override
         public Integer getInteger() {
-            throw new IllegalStateException();
+            throw new IllegalStateException("Could not call getInteger on list!");
         }
 
         @Override