add source code for stack

pom.xml

@@ -15,7 +15,7 @@
     <dependency>
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>
-      <version>4.12</version>
+      <version>4.13</version>
       <scope>test</scope>
     </dependency>
   </dependencies>

src/main/java/com/spring2go/algorithms/datastructures/linkedlist/DoublyLinkedList.java

@@ -3,6 +3,10 @@
 import java.util.Iterator;
 
 /**
+ * 双向链表实现.
+ *
+ * 用你的实现代码替换YOUR CODE HERE.
+ *
  * Created on Jun, 2020 by @author bobo
  */
 public class DoublyLinkedList<T> implements Iterable<T> {
@@ -88,6 +92,7 @@ public T removeLast() {
     }
 
     // 移除链表中的一个指定的节点，O(1)
+    // 内部使用
     private T remove(Node<T> node) {
         // YOUR CODE HERE
         return null;

src/main/java/com/spring2go/algorithms/datastructures/stack/ListStack.java

@@ -0,0 +1,61 @@
+package com.spring2go.algorithms.datastructures.stack;
+
+import java.util.Iterator;
+
+/**
+ * 基于链表实现的栈.
+ *
+ * 用你的实现代码替换YOUR CODE HERE.
+ *
+ * Created on Jun, 2020 by @author bobo
+ */
+public class ListStack<T> implements Iterable<T> {
+
+    private java.util.LinkedList<T> list = new java.util.LinkedList<>();
+
+    // 创建一个空栈
+    public ListStack() {}
+
+    // 创建一个带有一个初始元素的栈
+    public ListStack(T firstElem) {
+        // YOUR CODE HERE
+    }
+
+    // 返回栈中的元素个数
+    public int size() {
+        // YOUR CODE HERE
+        return 0;
+    }
+
+    // 检查栈是否为空
+    public boolean isEmpty() {
+        // YOUR CODE HERE
+        return false;
+    }
+
+    // 从栈中弹出一个元素
+    // 如果栈空就抛出一个异常
+    public T pop() {
+        // YOUR CODE HERE
+        return null;
+    }
+
+    // 查看栈顶元素(并不移除)
+    // 如果栈空就抛出一个异常
+    public T peek() {
+        // YOUR CODE HERE
+        return null;
+    }
+
+    // 将一个元素入栈
+    public void push(T elem) {
+        // YOUR CODE HERE
+    }
+
+    // 支持以迭代器方式对栈进行遍历
+    @Override
+    public Iterator<T> iterator() {
+        // YOUR CODE HERE
+        return null;
+    }
+}

src/test/java/com/spring2go/algorithms/datastructures/stack/StackTest.java

@@ -0,0 +1,67 @@
+package com.spring2go.algorithms.datastructures.stack;
+
+/**
+ * Created on Jun, 2020 by @author bobo
+ */
+import static org.junit.Assert.*;
+
+import org.junit.Test;
+
+public class StackTest {
+
+    private ListStack<Integer> stack = new ListStack<>();
+
+    @Test
+    public void testEmptyStack() {
+        assertTrue(stack.isEmpty());
+        assertEquals(stack.size(), 0);
+    }
+
+    @Test
+    public void testPopOnEmpty() {
+        assertThrows(Exception.class, stack::pop);
+    }
+
+    @Test
+    public void testPeekOnEmpty() {
+        assertThrows(Exception.class, stack::peek);
+    }
+
+    @Test
+    public void testPush() {
+        stack.push(2);
+        assertEquals(stack.size(), 1);
+    }
+
+    @Test
+    public void testPeek() {
+        stack.push(2);
+        assertEquals(2, (int) (Integer) stack.peek());
+        assertEquals(stack.size(), 1);
+    }
+
+    @Test
+    public void testPop() {
+        stack.push(2);
+        assertEquals(2, (int) stack.pop());
+        assertEquals(stack.size(), 0);
+    }
+
+    @Test
+    public void testExhaustively() {
+        assertTrue(stack.isEmpty());
+        stack.push(1);
+        assertFalse(stack.isEmpty());
+        stack.push(2);
+        assertEquals(stack.size(), 2);
+        assertEquals(2, (int) stack.peek());
+        assertEquals(stack.size(), 2);
+        assertEquals(2, (int) stack.pop());
+        assertEquals(stack.size(), 1);
+        assertEquals(1, (int) stack.peek());
+        assertEquals(stack.size(), 1);
+        assertEquals(1, (int) stack.pop());
+        assertEquals(stack.size(), 0);
+        assertTrue(stack.isEmpty());
+    }
+}