feat: adds Kimberlee's lesson 12 Stack.java file

Author: haldanek

lesson_12/structs_java/structs_app/src/main/java/com/codedifferently/lesson12/Stack.java

@@ -10,17 +10,29 @@ public Stack() {
 
   public void push(int value) {
     // Your code here
+    ListNode newNode = ListNode(value);
+    newNode.next = top;  
+    top = newNode;  
   }
 
   public int pop() {
-    return 0;
+    if (isEmpty()) {
+      throw new RuntimeException("Stack is empty! Cannot pop.");
+    }
+    int value = top.val;  
+    top = top.next;  
+    return value;   
   }
 
   public int peek() {
-    return 0;
+    if (isEmpty()) {
+      throw new RuntimeException("Stack is empty! Cannot peek.");
+    }
+    return top.val;  
   }
-
+  
   public boolean isEmpty() {
-    return true;
+    return top == null;  
   }
 }
+