diff --git a/src/data-structures/hash-table/HashTable.js b/src/data-structures/hash-table/HashTable.js
index f8fec1cd..d241dfb8 100644
--- a/src/data-structures/hash-table/HashTable.js
+++ b/src/data-structures/hash-table/HashTable.js
@@ -21,16 +21,32 @@ export default class HashTable {
 
   insert(key, value) {
     const bucketLinkedList = this.buckets[this.hash(key)];
-    bucketLinkedList.appendUnique({ key, value });
+    const node = bucketLinkedList.find({ callback: nodeValue => nodeValue.key === key });
+
+    if (!node) {
+      // Insert new node.
+      bucketLinkedList.append({ key, value });
+    } else {
+      // Update value of existing node.
+      node.value.value = value;
+    }
   }
 
   delete(key) {
     const bucketLinkedList = this.buckets[this.hash(key)];
-    return bucketLinkedList.deleteByKey(key);
+    const node = bucketLinkedList.find({ callback: nodeValue => nodeValue.key === key });
+
+    if (node) {
+      return bucketLinkedList.delete(node.value);
+    }
+
+    return null;
   }
 
   get(key) {
     const bucketLinkedList = this.buckets[this.hash(key)];
-    return bucketLinkedList.findByKey(key);
+    const node = bucketLinkedList.find({ callback: nodeValue => nodeValue.key === key });
+
+    return node ? node.value.value : null;
   }
 }
diff --git a/src/data-structures/hash-table/__test__/HashTable.test.js b/src/data-structures/hash-table/__test__/HashTable.test.js
index 71e18cb8..a39fcd24 100644
--- a/src/data-structures/hash-table/__test__/HashTable.test.js
+++ b/src/data-structures/hash-table/__test__/HashTable.test.js
@@ -31,19 +31,34 @@ describe('HashTable', () => {
     hashTable.insert('c', 'earth');
     hashTable.insert('d', 'ocean');
 
-    expect(hashTable.buckets[0].toString()).toBe('c:earth');
-    expect(hashTable.buckets[1].toString()).toBe('a:sky,d:ocean');
-    expect(hashTable.buckets[2].toString()).toBe('b:sea');
+    const stringifier = value => `${value.key}:${value.value}`;
 
-    expect(hashTable.get('a').value).toBe('sky');
-    expect(hashTable.get('d').value).toBe('ocean');
+    expect(hashTable.buckets[0].toString(stringifier)).toBe('c:earth');
+    expect(hashTable.buckets[1].toString(stringifier)).toBe('a:sky,d:ocean');
+    expect(hashTable.buckets[2].toString(stringifier)).toBe('b:sea');
+
+    expect(hashTable.get('a')).toBe('sky');
+    expect(hashTable.get('d')).toBe('ocean');
 
     hashTable.delete('a');
 
+    expect(hashTable.delete('not-existing')).toBeNull();
+
     expect(hashTable.get('a')).toBeNull();
-    expect(hashTable.get('d').value).toBe('ocean');
+    expect(hashTable.get('d')).toBe('ocean');
 
     hashTable.insert('d', 'ocean-new');
-    expect(hashTable.get('d').value).toBe('ocean-new');
+    expect(hashTable.get('d')).toBe('ocean-new');
+  });
+
+  it('should be possible to add objects to hash table', () => {
+    const hashTable = new HashTable();
+
+    hashTable.insert('objectKey', { prop1: 'a', prop2: 'b' });
+
+    const object = hashTable.get('objectKey');
+    expect(object).toBeDefined();
+    expect(object.prop1).toBe('a');
+    expect(object.prop2).toBe('b');
   });
 });
diff --git a/src/data-structures/linked-list/LinkedList.js b/src/data-structures/linked-list/LinkedList.js
index f8968aaa..d4ab790c 100644
--- a/src/data-structures/linked-list/LinkedList.js
+++ b/src/data-structures/linked-list/LinkedList.js
@@ -2,74 +2,39 @@ import LinkedListNode from './LinkedListNode';
 
 export default class LinkedList {
   constructor() {
+    /** @var LinkedListNode */
     this.head = null;
+
+    /** @var LinkedListNode */
     this.tail = null;
   }
 
-  prepend({ value, key = null }) {
-    const newNode = new LinkedListNode({ value, key, next: this.head });
-
+  prepend(value) {
     // Make new node to be a head.
-    this.head = newNode;
+    this.head = new LinkedListNode(value, this.head);
 
-    return newNode;
+    return this;
   }
 
-  append({ value, key = null }) {
-    const newNode = new LinkedListNode({ value, key });
+  append(value) {
+    const newNode = new LinkedListNode(value);
 
     // If there is no head yet let's make new node a head.
     if (!this.head) {
       this.head = newNode;
       this.tail = newNode;
 
-      return newNode;
+      return this;
     }
 
     // Attach new node to the end of linked list.
     this.tail.next = newNode;
     this.tail = newNode;
 
-    return newNode;
+    return this;
   }
 
-  appendUnique({ value, key = null }) {
-    const newNode = new LinkedListNode({ value, key });
-
-    // If there is no head yet let's make new node a head.
-    if (!this.head) {
-      this.head = newNode;
-      this.tail = newNode;
-
-      return newNode;
-    }
-
-    // Rewind to last node.
-    let currentNode = this.head;
-    while (currentNode.next !== null) {
-      // If there is a node with specified key exists then update it instead of adding new one.
-      if (key && currentNode.key === key) {
-        currentNode.value = value;
-        return currentNode;
-      }
-
-      currentNode = currentNode.next;
-    }
-
-    // If there is a node with specified key exists then update it instead of adding new one.
-    if (key && currentNode.key === key) {
-      currentNode.value = value;
-      return currentNode;
-    }
-
-    // Attach new node to the end of linked list.
-    currentNode.next = newNode;
-    this.tail = newNode;
-
-    return newNode;
-  }
-
-  deleteByValue(value) {
+  delete(value) {
     if (!this.head) {
       return null;
     }
@@ -102,37 +67,28 @@ export default class LinkedList {
     return deletedNode;
   }
 
-  deleteByKey(key) {
+  find({ value = undefined, callback = undefined }) {
     if (!this.head) {
       return null;
     }
 
-    let deletedNode = null;
-
-    // If the head must be deleted then make 2nd node to be a head.
-    if (this.head.key === key) {
-      deletedNode = this.head;
-      this.head = this.head.next;
-    }
-
     let currentNode = this.head;
 
-    // If next node must be deleted then make next node to be a next next one.
-    while (currentNode.next) {
-      if (currentNode.next.key === key) {
-        deletedNode = currentNode.next;
-        currentNode.next = currentNode.next.next;
-      } else {
-        currentNode = currentNode.next;
+    while (currentNode) {
+      // If callback is specified then try to find node by callback.
+      if (callback && callback(currentNode.value)) {
+        return currentNode;
       }
+
+      // If value is specified then try to compare by value..
+      if (value !== undefined && currentNode.value === value) {
+        return currentNode;
+      }
+
+      currentNode = currentNode.next;
     }
 
-    // Check if tail must be deleted.
-    if (this.tail.key === key) {
-      this.tail = currentNode;
-    }
-
-    return deletedNode;
+    return null;
   }
 
   deleteTail() {
@@ -177,32 +133,15 @@ export default class LinkedList {
     return deletedHead;
   }
 
-  findByKey(key) {
-    let currentNode = this.head;
+  toString(callback) {
+    const nodeStrings = [];
 
+    let currentNode = this.head;
     while (currentNode) {
-      if (currentNode.key === key) {
-        return currentNode;
-      }
+      nodeStrings.push(currentNode.toString(callback));
       currentNode = currentNode.next;
     }
 
-    return null;
-  }
-
-  toArray() {
-    const listArray = [];
-    let currentNode = this.head;
-
-    while (currentNode) {
-      listArray.push(currentNode.toString());
-      currentNode = currentNode.next;
-    }
-
-    return listArray;
-  }
-
-  toString() {
-    return this.toArray().toString();
+    return nodeStrings.toString();
   }
 }
diff --git a/src/data-structures/linked-list/LinkedListNode.js b/src/data-structures/linked-list/LinkedListNode.js
index 58d900db..d136e863 100644
--- a/src/data-structures/linked-list/LinkedListNode.js
+++ b/src/data-structures/linked-list/LinkedListNode.js
@@ -1,17 +1,10 @@
 export default class LinkedListNode {
-  constructor({ value, next = null, key = null }) {
+  constructor(value, next = null) {
     this.value = value;
     this.next = next;
-
-    // Key is added to make this linked list nodes to be reusable in hash tables.
-    this.key = key;
   }
 
-  toString() {
-    if (this.key) {
-      return `${this.key}:${this.value}`;
-    }
-
-    return `${this.value}`;
+  toString(callback) {
+    return callback ? callback(this.value) : `${this.value}`;
   }
 }
diff --git a/src/data-structures/linked-list/__test__/LinkedList.test.js b/src/data-structures/linked-list/__test__/LinkedList.test.js
index 2806e68e..3f877f83 100644
--- a/src/data-structures/linked-list/__test__/LinkedList.test.js
+++ b/src/data-structures/linked-list/__test__/LinkedList.test.js
@@ -12,30 +12,17 @@ describe('LinkedList', () => {
     expect(linkedList.head).toBeNull();
     expect(linkedList.tail).toBeNull();
 
-    const node1 = linkedList.append({ value: 1 });
-    const node2 = linkedList.append({ value: 2, key: 'test' });
+    linkedList.append(1);
+    linkedList.append(2);
 
-    expect(node1.value).toBe(1);
-    expect(node2.value).toBe(2);
-    expect(node2.key).toBe('test');
-
-    expect(linkedList.head.toString()).toBe('1');
-    expect(linkedList.tail.toString()).toBe('test:2');
-
-    expect(linkedList.toString()).toBe('1,test:2');
+    expect(linkedList.toString()).toBe('1,2');
   });
 
   it('should prepend node to linked list', () => {
     const linkedList = new LinkedList();
 
-    const node1 = linkedList.append({ value: 1 });
-    const node2 = linkedList.prepend({ value: 2 });
-
-    expect(node1.value).toBe(1);
-    expect(node2.value).toBe(2);
-
-    expect(linkedList.head.toString()).toBe('2');
-    expect(linkedList.tail.toString()).toBe('1');
+    linkedList.append(1);
+    linkedList.prepend(2);
 
     expect(linkedList.toString()).toBe('2,1');
   });
@@ -43,74 +30,51 @@ describe('LinkedList', () => {
   it('should delete node by value from linked list', () => {
     const linkedList = new LinkedList();
 
-    expect(linkedList.deleteByValue(5)).toBeNull();
+    expect(linkedList.delete(5)).toBeNull();
 
-    linkedList.append({ value: 1 });
-    linkedList.append({ value: 2 });
-    linkedList.append({ value: 3 });
-    linkedList.append({ value: 3 });
-    linkedList.append({ value: 3 });
-    linkedList.append({ value: 4 });
-    linkedList.append({ value: 5 });
+    linkedList.append(1);
+    linkedList.append(2);
+    linkedList.append(3);
+    linkedList.append(3);
+    linkedList.append(3);
+    linkedList.append(4);
+    linkedList.append(5);
 
     expect(linkedList.head.toString()).toBe('1');
     expect(linkedList.tail.toString()).toBe('5');
 
-    const deletedNode = linkedList.deleteByValue(3);
+    const deletedNode = linkedList.delete(3);
     expect(deletedNode.value).toBe(3);
     expect(linkedList.toString()).toBe('1,2,4,5');
 
-    linkedList.deleteByValue(3);
+    linkedList.delete(3);
     expect(linkedList.toString()).toBe('1,2,4,5');
 
-    linkedList.deleteByValue(1);
+    linkedList.delete(1);
     expect(linkedList.toString()).toBe('2,4,5');
 
     expect(linkedList.head.toString()).toBe('2');
     expect(linkedList.tail.toString()).toBe('5');
 
-    linkedList.deleteByValue(5);
+    linkedList.delete(5);
     expect(linkedList.toString()).toBe('2,4');
 
     expect(linkedList.head.toString()).toBe('2');
     expect(linkedList.tail.toString()).toBe('4');
 
-    linkedList.deleteByValue(4);
+    linkedList.delete(4);
     expect(linkedList.toString()).toBe('2');
 
     expect(linkedList.head.toString()).toBe('2');
     expect(linkedList.tail.toString()).toBe('2');
   });
 
-  it('should delete node by key from linked list', () => {
-    const linkedList = new LinkedList();
-
-    expect(linkedList.deleteByKey('key')).toBeNull();
-
-    linkedList.append({ value: 1, key: 'test1' });
-    linkedList.append({ value: 2, key: 'test2' });
-    linkedList.append({ value: 3, key: 'test3' });
-    linkedList.append({ value: 4, key: 'test4' });
-
-    const deletedNode1 = linkedList.deleteByKey('test2');
-    expect(deletedNode1.key).toBe('test2');
-    expect(linkedList.toString()).toBe('test1:1,test3:3,test4:4');
-
-    const deletedNode2 = linkedList.deleteByKey('test1');
-    expect(deletedNode2.key).toBe('test1');
-    expect(linkedList.toString()).toBe('test3:3,test4:4');
-
-    const deletedNode3 = linkedList.deleteByKey('test4');
-    expect(deletedNode3.key).toBe('test4');
-    expect(linkedList.toString()).toBe('test3:3');
-  });
-
   it('should delete linked list tail', () => {
     const linkedList = new LinkedList();
 
-    linkedList.append({ value: 1 });
-    linkedList.append({ value: 2 });
-    linkedList.append({ value: 3 });
+    linkedList.append(1);
+    linkedList.append(2);
+    linkedList.append(3);
 
     expect(linkedList.head.toString()).toBe('1');
     expect(linkedList.tail.toString()).toBe('3');
@@ -142,8 +106,8 @@ describe('LinkedList', () => {
 
     expect(linkedList.deleteHead()).toBeNull();
 
-    linkedList.append({ value: 1 });
-    linkedList.append({ value: 2 });
+    linkedList.append(1);
+    linkedList.append(2);
 
     expect(linkedList.head.toString()).toBe('1');
     expect(linkedList.tail.toString()).toBe('2');
@@ -163,26 +127,52 @@ describe('LinkedList', () => {
     expect(linkedList.tail).toBeNull();
   });
 
-  it('should append unique nodes', () => {
+  it('should be possible to store objects in the list and to print them out', () => {
     const linkedList = new LinkedList();
 
-    linkedList.appendUnique({ value: 1, key: 'test1' });
-    linkedList.appendUnique({ value: 2, key: 'test2' });
-    linkedList.appendUnique({ value: 3, key: 'test2' });
-    linkedList.appendUnique({ value: 5, key: 'test1' });
+    const nodeValue1 = { value: 1, key: 'key1' };
+    const nodeValue2 = { value: 2, key: 'key2' };
 
-    expect(linkedList.toString()).toBe('test1:5,test2:3');
+    linkedList
+      .append(nodeValue1)
+      .prepend(nodeValue2);
+
+    const nodeStringifier = value => `${value.key}:${value.value}`;
+
+    expect(linkedList.toString(nodeStringifier)).toBe('key2:2,key1:1');
   });
 
-  it('should find node by its key', () => {
+  it('should find node by value', () => {
     const linkedList = new LinkedList();
 
-    expect(linkedList.findByKey('test')).toBeNull();
+    expect(linkedList.find({ value: 5 })).toBeNull();
 
-    linkedList.appendUnique({ value: 1, key: 'test1' });
-    linkedList.appendUnique({ value: 2, key: 'test2' });
-    linkedList.appendUnique({ value: 3, key: 'test3' });
+    linkedList.append(1);
+    expect(linkedList.find({ value: 1 })).toBeDefined();
 
-    expect(linkedList.findByKey('test3').toString()).toBe('test3:3');
+    linkedList
+      .append(2)
+      .append(3);
+
+    const node = linkedList.find({ value: 2 });
+
+    expect(node.value).toBe(2);
+    expect(linkedList.find({ value: 5 })).toBeNull();
+  });
+
+  it('should find node by callback', () => {
+    const linkedList = new LinkedList();
+
+    linkedList
+      .append({ value: 1, key: 'test1' })
+      .append({ value: 2, key: 'test2' })
+      .append({ value: 3, key: 'test3' });
+
+    const node = linkedList.find({ callback: value => value.key === 'test2' });
+
+    expect(node).toBeDefined();
+    expect(node.value.value).toBe(2);
+    expect(node.value.key).toBe('test2');
+    expect(linkedList.find({ callback: value => value.key === 'test5' })).toBeNull();
   });
 });
diff --git a/src/data-structures/linked-list/__test__/LinkedListNode.test.js b/src/data-structures/linked-list/__test__/LinkedListNode.test.js
index 19bf3c9b..0a989fc8 100644
--- a/src/data-structures/linked-list/__test__/LinkedListNode.test.js
+++ b/src/data-structures/linked-list/__test__/LinkedListNode.test.js
@@ -1,18 +1,46 @@
 import LinkedListNode from '../LinkedListNode';
 
 describe('LinkedListNode', () => {
-  it('should create list node with kay and value', () => {
-    const node = new LinkedListNode({ value: 1, key: 'test' });
+  it('should create list node with value', () => {
+    const node = new LinkedListNode(1);
+
     expect(node.value).toBe(1);
-    expect(node.key).toBe('test');
     expect(node.next).toBeNull();
   });
 
+  it('should create list node with object as a value', () => {
+    const nodeValue = { value: 1, key: 'test' };
+    const node = new LinkedListNode(nodeValue);
+
+    expect(node.value.value).toBe(1);
+    expect(node.value.key).toBe('test');
+    expect(node.next).toBeNull();
+  });
+
+  it('should link nodes together', () => {
+    const node2 = new LinkedListNode(2);
+    const node1 = new LinkedListNode(1, node2);
+
+    expect(node1.next).toBeDefined();
+    expect(node2.next).toBeNull();
+    expect(node1.value).toBe(1);
+    expect(node1.next.value).toBe(2);
+  });
+
   it('should convert node to string', () => {
-    const node = new LinkedListNode({ value: 1 });
-    const nodeWithKey = new LinkedListNode({ value: 1, key: 'test' });
+    const node = new LinkedListNode(1);
 
     expect(node.toString()).toBe('1');
-    expect(nodeWithKey.toString()).toBe('test:1');
+
+    node.value = 'string value';
+    expect(node.toString()).toBe('string value');
+  });
+
+  it('should convert node to string with custom stringifier', () => {
+    const nodeValue = { value: 1, key: 'test' };
+    const node = new LinkedListNode(nodeValue);
+    const toStringCallback = value => `value: ${value.value}, key: ${value.key}`;
+
+    expect(node.toString(toStringCallback)).toBe('value: 1, key: test');
   });
 });
diff --git a/src/data-structures/queue/Queue.js b/src/data-structures/queue/Queue.js
index b2d50458..49332400 100644
--- a/src/data-structures/queue/Queue.js
+++ b/src/data-structures/queue/Queue.js
@@ -18,11 +18,15 @@ export default class Queue {
   }
 
   enqueue(value) {
-    this.linkedList.append({ value });
+    this.linkedList.append(value);
   }
 
   dequeue() {
     const removedHead = this.linkedList.deleteHead();
     return removedHead ? removedHead.value : null;
   }
+
+  toString(callback) {
+    return this.linkedList.toString(callback);
+  }
 }
diff --git a/src/data-structures/queue/__test__/Queue.test.js b/src/data-structures/queue/__test__/Queue.test.js
index 587e37b5..3e0fad8c 100644
--- a/src/data-structures/queue/__test__/Queue.test.js
+++ b/src/data-structures/queue/__test__/Queue.test.js
@@ -13,7 +13,20 @@ describe('Queue', () => {
     queue.enqueue(1);
     queue.enqueue(2);
 
-    expect(queue.linkedList.toString()).toBe('1,2');
+    expect(queue.toString()).toBe('1,2');
+  });
+
+  it('should be possible to enqueue/dequeue objects', () => {
+    const queue = new Queue();
+
+    queue.enqueue({ value: 'test1', key: 'key1' });
+    queue.enqueue({ value: 'test2', key: 'key2' });
+
+    const stringifier = value => `${value.key}:${value.value}`;
+
+    expect(queue.toString(stringifier)).toBe('key1:test1,key2:test2');
+    expect(queue.dequeue().value).toBe('test1');
+    expect(queue.dequeue().value).toBe('test2');
   });
 
   it('should peek data from queue', () => {
diff --git a/src/data-structures/stack/Stack.js b/src/data-structures/stack/Stack.js
index 3c2b0c45..665bec2e 100644
--- a/src/data-structures/stack/Stack.js
+++ b/src/data-structures/stack/Stack.js
@@ -18,11 +18,15 @@ export default class Stack {
   }
 
   push(value) {
-    this.linkedList.append({ value });
+    this.linkedList.append(value);
   }
 
   pop() {
     const removedTail = this.linkedList.deleteTail();
     return removedTail ? removedTail.value : null;
   }
+
+  toString(callback) {
+    return this.linkedList.toString(callback);
+  }
 }
diff --git a/src/data-structures/stack/__test__/Stack.test.js b/src/data-structures/stack/__test__/Stack.test.js
index 63978243..291bc430 100644
--- a/src/data-structures/stack/__test__/Stack.test.js
+++ b/src/data-structures/stack/__test__/Stack.test.js
@@ -13,7 +13,7 @@ describe('Stack', () => {
     stack.push(1);
     stack.push(2);
 
-    expect(stack.linkedList.toString()).toBe('1,2');
+    expect(stack.toString()).toBe('1,2');
   });
 
   it('should peek data from stack', () => {
@@ -49,4 +49,17 @@ describe('Stack', () => {
     expect(stack.pop()).toBeNull();
     expect(stack.isEmpty()).toBeTruthy();
   });
+
+  it('should be possible to push/pop objects', () => {
+    const stack = new Stack();
+
+    stack.push({ value: 'test1', key: 'key1' });
+    stack.push({ value: 'test2', key: 'key2' });
+
+    const stringifier = value => `${value.key}:${value.value}`;
+
+    expect(stack.toString(stringifier)).toBe('key1:test1,key2:test2');
+    expect(stack.pop().value).toBe('test2');
+    expect(stack.pop().value).toBe('test1');
+  });
 });