feat: Adds game results & stack

Author: Dasiame

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

@@ -9,18 +9,25 @@ public Stack() {
   }
 
   public void push(int value) {
-    // Your code here
+    ListNode first = new ListNode(value);
+    first.next = top;
+    top = first;
   }
 
   public int pop() {
-    return 0;
+    if (isEmpty()) {
+      return -1;
+    }
+    int value = top.val;
+    top = top.next;
+    return value;
   }
 
   public int peek() {
-    return 0;
+    return top.val;
   }
 
   public boolean isEmpty() {
-    return true;
+    return top == null;
   }
 }