Add Stack.

src/data-structures/stack/README.md

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+# Queue

src/data-structures/stack/Stack.js

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+import LinkedList from '../linked-list/LinkedList';
+
+export default class Stack {
+  constructor() {
+    this.linkedList = new LinkedList();
+  }
+
+  isEmpty() {
+    return !this.linkedList.tail;
+  }
+
+  peek() {
+    if (!this.linkedList.tail) {
+      return null;
+    }
+
+    return this.linkedList.tail.value;
+  }
+
+  push(value) {
+    this.linkedList.append({ value });
+  }
+
+  pop() {
+    const removedTail = this.linkedList.deleteTail();
+    return removedTail ? removedTail.value : null;
+  }
+}

src/data-structures/stack/__test__/Stack.test.js

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+import Stack from '../Stack';
+
+describe('Stack', () => {
+  it('should create empty stack', () => {
+    const stack = new Stack();
+    expect(stack).not.toBeNull();
+    expect(stack.linkedList).not.toBeNull();
+  });
+
+  it('should stack data to stack', () => {
+    const stack = new Stack();
+
+    stack.push(1);
+    stack.push(2);
+
+    expect(stack.linkedList.toString()).toBe('1,2');
+  });
+
+  it('should peek data from stack', () => {
+    const stack = new Stack();
+
+    expect(stack.peek()).toBeNull();
+
+    stack.push(1);
+    stack.push(2);
+
+    expect(stack.peek()).toBe(2);
+    expect(stack.peek()).toBe(2);
+  });
+
+  it('should check if stack is empty', () => {
+    const stack = new Stack();
+
+    expect(stack.isEmpty()).toBeTruthy();
+
+    stack.push(1);
+
+    expect(stack.isEmpty()).toBeFalsy();
+  });
+
+  it('should pop data from stack', () => {
+    const stack = new Stack();
+
+    stack.push(1);
+    stack.push(2);
+
+    expect(stack.pop()).toBe(2);
+    expect(stack.pop()).toBe(1);
+    expect(stack.isEmpty()).toBeTruthy();
+  });
+});