diff --git a/data_structures/stack.h b/data_structures/stack.h
index f4b8992e7..483648046 100644
--- a/data_structures/stack.h
+++ b/data_structures/stack.h
@@ -1,18 +1,27 @@
-/* This class specifies the basic operation on a stack as a linked list */
+/**
+ * @file stack.h
+ * @author danghai
+ * @brief  This class specifies the basic operation on a stack as a linked list
+ **/
 #ifndef DATA_STRUCTURES_STACK_H_
 #define DATA_STRUCTURES_STACK_H_
 
 #include <cassert>
 #include <iostream>
 
-/* Definition of the node */
+/** Definition of the node as a linked-list
+ * \tparam Type type of data nodes of the linked list should contain
+ */
 template <class Type>
 struct node {
-    Type data;
-    node<Type> *next;
+    Type data;         ///< data at current node
+    node<Type> *next;  ///< pointer to the next ::node instance
 };
 
-/* Definition of the stack class */
+/** Definition of the stack class
+ * \tparam Type type of data nodes of the linked list in the stack should
+ * contain
+ */
 template <class Type>
 class stack {
  public:
@@ -20,7 +29,7 @@ class stack {
     void display() {
         node<Type> *current = stackTop;
         std::cout << "Top --> ";
-        while (current != NULL) {
+        while (current != nullptr) {
             std::cout << current->data << "  ";
             current = current->next;
         }
@@ -30,15 +39,45 @@ class stack {
 
     /** Default constructor*/
     stack() {
-        stackTop = NULL;
+        stackTop = nullptr;
         size = 0;
     }
 
+    /** Copy constructor*/
+    explicit stack(const stack<Type> &otherStack) {
+        node<Type> *newNode, *current, *last;
+
+        /* If stack is no empty, make it empty */
+        if (stackTop != nullptr) {
+            stackTop = nullptr;
+        }
+        if (otherStack.stackTop == nullptr) {
+            stackTop = nullptr;
+        } else {
+            current = otherStack.stackTop;
+            stackTop = new node<Type>;
+            stackTop->data = current->data;
+            stackTop->next = nullptr;
+            last = stackTop;
+            current = current->next;
+            /* Copy the remaining stack */
+            while (current != nullptr) {
+                newNode = new node<Type>;
+                newNode->data = current->data;
+                newNode->next = nullptr;
+                last->next = newNode;
+                last = newNode;
+                current = current->next;
+            }
+        }
+        size = otherStack.size;
+    }
+
     /** Destructor */
     ~stack() {}
 
     /** Determine whether the stack is empty */
-    bool isEmptyStack() { return (stackTop == NULL); }
+    bool isEmptyStack() { return (stackTop == nullptr); }
 
     /** Add new item to the stack */
     void push(Type item) {
@@ -52,7 +91,7 @@ class stack {
 
     /** Return the top element of the stack */
     Type top() {
-        assert(stackTop != NULL);
+        assert(stackTop != nullptr);
         return stackTop->data;
     }
 
@@ -70,30 +109,30 @@ class stack {
     }
 
     /** Clear stack */
-    void clear() { stackTop = NULL; }
+    void clear() { stackTop = nullptr; }
 
     /** Overload "=" the assignment operator */
     stack<Type> &operator=(const stack<Type> &otherStack) {
         node<Type> *newNode, *current, *last;
 
         /* If stack is no empty, make it empty */
-        if (stackTop != NULL) {
-            stackTop = NULL;
+        if (stackTop != nullptr) {
+            stackTop = nullptr;
         }
-        if (otherStack.stackTop == NULL) {
-            stackTop = NULL;
+        if (otherStack.stackTop == nullptr) {
+            stackTop = nullptr;
         } else {
             current = otherStack.stackTop;
             stackTop = new node<Type>;
             stackTop->data = current->data;
-            stackTop->next = NULL;
+            stackTop->next = nullptr;
             last = stackTop;
             current = current->next;
             /* Copy the remaining stack */
-            while (current != NULL) {
+            while (current != nullptr) {
                 newNode = new node<Type>;
                 newNode->data = current->data;
-                newNode->next = NULL;
+                newNode->next = nullptr;
                 last->next = newNode;
                 last = newNode;
                 current = current->next;
@@ -105,7 +144,7 @@ class stack {
 
  private:
     node<Type> *stackTop; /**< Pointer to the stack */
-    int size;
+    int size;             ///< size of stack
 };
 
 #endif  // DATA_STRUCTURES_STACK_H_