Update array_stack.cpp

Make `vec` a private member.

codes/cpp/chapter_stack_and_queue/array_stack.cpp

@@ -8,9 +8,9 @@
 
 /* 基于数组实现的栈 */
 class ArrayStack {
-public:
+private:
     vector<int> vec;
-
+public:
     /* 获取栈的长度 */
     int size() {
         return vec.size();
@@ -37,8 +37,14 @@ public:
     int top() {
         return vec.back();
     }
+
+    /* 返回 Vector */
+    vector<int> toVector() {
+        return vec;
+    }
 };
 
+
 /* Driver Code */
 int main() {
     /* 初始化栈 */
@@ -51,7 +57,8 @@ int main() {
     stack->push(5);
     stack->push(4);
     cout << "栈 stack = ";
-    PrintUtil::printVector(stack->vec);
+    vector<int> vec = stack->toVector();
+    PrintUtil::printVector(vec);
 
     /* 访问栈顶元素 */
     int top = stack->top();
@@ -60,7 +67,8 @@ int main() {
     /* 元素出栈 */
     int pop = stack->pop();
     cout << "出栈元素 pop = " << pop << "，出栈后 stack = ";
-    PrintUtil::printVector(stack->vec);
+    vec = stack->toVector();
+    PrintUtil::printVector(vec);
 
     /* 获取栈的长度 */
     int size = stack->size();