update stack and queue.

codes/cpp/chapter_stack_and_queue/array_stack.cpp

@@ -69,6 +69,10 @@ int main() {
     int top = stack->top();
     cout << "栈顶元素 top = " << top << endl;
 
+    /* 访问索引 index 处元素 */
+    int num = stack->get(3);
+    cout << "栈索引 3 处的元素为 num = " << num << endl;
+
     /* 元素出栈 */
     int pop = stack->pop();
     cout << "出栈元素 pop = " << pop << "，出栈后 stack = ";