update stack and queue.

codes/java/chapter_stack_and_queue/array_stack.java

@@ -69,6 +69,10 @@ public class array_stack {
         int peek = stack.peek();
         System.out.println("栈顶元素 peek = " + peek);
 
+        /* 访问索引 index 处元素 */
+        int num = stack.get(3);
+        System.out.println("栈索引 3 处的元素为 num = " + num);
+
         /* 元素出栈 */
         int pop = stack.pop();
         System.out.println("出栈元素 pop = " + pop + "，出栈后 stack = " + Arrays.toString(stack.toArray()));