Fixed iterator in ListStack to return elements in LIFO order (#1239)

Co-authored-by: Dhruv Gupta <dhruv.gupta01@nagarro.com>

Author: dhruvgupta77

src/main/java/com/williamfiset/algorithms/datastructures/stack/ListStack.java

@@ -55,6 +55,6 @@ public int search(T elem) {
   // Allow users to iterate through the stack using an iterator
   @Override
   public java.util.Iterator<T> iterator() {
-    return list.iterator();
+    return list.descendingIterator();
   }
 }

src/test/java/com/williamfiset/algorithms/datastructures/stack/StackTest.java

@@ -5,6 +5,7 @@
 
 import java.util.ArrayList;
 import java.util.List;
+import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.MethodSource;
 
@@ -78,4 +79,33 @@ public void testExhaustively(Stack<Integer> stack) {
     assertThat(stack.size()).isEqualTo(0);
     assertThat(stack.isEmpty()).isTrue();
   }
+
+  @Test
+  public void testIteratorLifoOrder() {
+    ListStack<Integer> stack = new ListStack<>();
+    stack.push(1);
+    stack.push(2);
+    stack.push(3);
+
+    List<Integer> iterated = new ArrayList<>();
+    for (Integer v : stack) iterated.add(v);
+
+    // Expect iteration from top (3) to bottom (1)
+    assertThat(iterated).containsExactly(3, 2, 1).inOrder();
+
+    // Iterator should not have removed elements
+    assertThat(stack.size()).isEqualTo(3);
+    assertThat(stack.peek()).isEqualTo(3);
+  }
+
+  @Test
+  public void testIteratorOnEmptyStack() {
+    ListStack<Integer> stack = new ListStack<>();
+
+    List<Integer> iterated = new ArrayList<>();
+    for (Integer v : stack) iterated.add(v);
+
+    assertThat(iterated).isEmpty();
+    assertThat(stack.isEmpty()).isTrue();
+  }
 }