style: enable FinalClass in checkstyle (#5154)

src/main/java/com/thealgorithms/datastructures/bags/Bag.java

@@ -13,7 +13,7 @@ public class Bag<Element> implements Iterable<Element> {
     private Node<Element> firstElement; // first element of the bag
     private int size; // size of bag
 
-    private static class Node<Element> {
+    private static final class Node<Element> {
 
         private Element content;
         private Node<Element> nextElement;

src/main/java/com/thealgorithms/datastructures/dynamicarray/DynamicArray.java

@@ -153,7 +153,7 @@ public class DynamicArray<E> implements Iterable<E> {
         return new DynamicArrayIterator();
     }
 
-    private class DynamicArrayIterator implements Iterator<E> {
+    private final class DynamicArrayIterator implements Iterator<E> {
 
         private int cursor;
 

src/main/java/com/thealgorithms/datastructures/graphs/WelshPowell.java

@@ -17,7 +17,7 @@ public final class WelshPowell {
     private WelshPowell() {
     }
 
-    static class Graph {
+    static final class Graph {
         private HashSet<Integer>[] adjacencyLists;
 
         private Graph(int vertices) {

src/main/java/com/thealgorithms/datastructures/hashmap/hashing/Intersection.java

@@ -12,7 +12,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
-public class Intersection {
+public final class Intersection {
 
     public static List<Integer> intersection(int[] arr1, int[] arr2) {
         if (arr1 == null || arr2 == null || arr1.length == 0 || arr2.length == 0) {

src/main/java/com/thealgorithms/datastructures/heaps/LeftistHeap.java

@@ -13,7 +13,7 @@ import java.util.ArrayList;
  */
 
 public class LeftistHeap {
-    private class Node {
+    private final class Node {
         private int element, npl;
         private Node left, right;
 

src/main/java/com/thealgorithms/datastructures/lists/CircleLinkedList.java

@@ -2,7 +2,7 @@ package com.thealgorithms.datastructures.lists;
 
 public class CircleLinkedList<E> {
 
-    private static class Node<E> {
+    private static final class Node<E> {
 
         Node<E> next;
         E value;

src/main/java/com/thealgorithms/datastructures/lists/Merge_K_SortedLinkedlist.java

@@ -43,7 +43,7 @@ public class Merge_K_SortedLinkedlist {
         return head;
     }
 
-    private class Node {
+    private final class Node {
 
         private int data;
         private Node next;

src/main/java/com/thealgorithms/datastructures/trees/GenericTree.java

@@ -16,7 +16,7 @@ import java.util.Scanner;
  */
 public class GenericTree {
 
-    private static class Node {
+    private static final class Node {
 
         int data;
         ArrayList<Node> child = new ArrayList<>();

src/main/java/com/thealgorithms/datastructures/trees/TreeRandomNode.java

@@ -26,7 +26,7 @@ import java.util.ArrayList;
 
 public class TreeRandomNode {
 
-    private class Node {
+    private final class Node {
 
         int item;
         Node left, right;