Add HashTable.

README.md

@@ -3,6 +3,7 @@
 ## Data Structures
 
 - [Linked List](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/linked-list)
+- [Hash Table](https://github.com/trekhleb/javascript-algorithms/tree/master/src/data-structures/hash-table)
 
 ## Running Tests
 

src/data-structures/hash-table/HashTable.js

@@ -0,0 +1,36 @@
+import LinkedList from '../linked-list/LinkedList';
+
+const defaultHashTableSize = 32;
+
+export default class HashTable {
+  constructor(hashTableSize = defaultHashTableSize) {
+    // Create hash table of certain size and fill each bucket with empty linked list.
+    this.buckets = Array(hashTableSize).fill(null).map(() => new LinkedList());
+  }
+
+  // Converts key string to hash number.
+  hash(key) {
+    const hash = Array.from(key).reduce(
+      (hashAccumulator, keySymbol) => (hashAccumulator + keySymbol.charCodeAt(0)),
+      0,
+    );
+
+    // Reduce hash number so it would fit hash table size.
+    return hash % this.buckets.length;
+  }
+
+  insert(key, value) {
+    const bucketLinkedList = this.buckets[this.hash(key)];
+    bucketLinkedList.appendUnique({ key, value });
+  }
+
+  delete(key) {
+    const bucketLinkedList = this.buckets[this.hash(key)];
+    return bucketLinkedList.deleteByKey(key);
+  }
+
+  get(key) {
+    const bucketLinkedList = this.buckets[this.hash(key)];
+    return bucketLinkedList.findByKey(key);
+  }
+}

src/data-structures/hash-table/README.md

@@ -0,0 +1,9 @@
+# Hashed Table
+
+|Operation                  |Complexity         |
+|---------------------------|-------------------|
+|Find                       |O(1)*              |
+|Insert                     |O(1)*              |
+|Delete                     |O(1)*              |
+
+* - assuming that we have "good" hash function and big enough hash table size so that collisions are rare. 

src/data-structures/hash-table/__test__/HashTable.test.js

@@ -0,0 +1,49 @@
+import HashTable from '../HashTable';
+
+describe('HashTable', () => {
+  it('should create hash table of certain size', () => {
+    const defaultHashTable = new HashTable();
+    expect(defaultHashTable.buckets.length).toBe(32);
+
+    const biggerHashTable = new HashTable(64);
+    expect(biggerHashTable.buckets.length).toBe(64);
+  });
+
+  it('should generate proper hash for specified keys', () => {
+    const hashTable = new HashTable();
+
+    expect(hashTable.hash('a')).toBe(1);
+    expect(hashTable.hash('b')).toBe(2);
+    expect(hashTable.hash('abc')).toBe(6);
+  });
+
+  it('should insert, read and delete data with collisions', () => {
+    const hashTable = new HashTable(3);
+
+    expect(hashTable.hash('a')).toBe(1);
+    expect(hashTable.hash('b')).toBe(2);
+    expect(hashTable.hash('c')).toBe(0);
+    expect(hashTable.hash('d')).toBe(1);
+
+    hashTable.insert('a', 'sky-old');
+    hashTable.insert('a', 'sky');
+    hashTable.insert('b', 'sea');
+    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');
+
+    expect(hashTable.get('a').value).toBe('sky');
+    expect(hashTable.get('d').value).toBe('ocean');
+
+    hashTable.delete('a');
+
+    expect(hashTable.get('a')).toBeNull();
+    expect(hashTable.get('d').value).toBe('ocean');
+
+    hashTable.insert('d', 'ocean-new');
+    expect(hashTable.get('d').value).toBe('ocean-new');
+  });
+});

src/data-structures/linked-list/LinkedList.js

@@ -5,8 +5,8 @@ export default class LinkedList {
     this.head = null;
   }
 
-  append(value) {
-    const newNode = new LinkedListNode(value);
+  append({ 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) {
@@ -27,20 +27,57 @@ export default class LinkedList {
     return newNode;
   }
 
-  prepend(value) {
-    const newNode = new LinkedListNode(value, this.head);
+  prepend({ value, key = null }) {
+    const newNode = new LinkedListNode({ value, key, next: this.head });
+
+    // Make new node to be a head.
     this.head = newNode;
 
     return newNode;
   }
 
-  delete(value) {
+  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;
+
+      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;
+
+    return newNode;
+  }
+
+  deleteByValue(value) {
     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.value === value) {
       deletedNode = this.head;
       this.head = this.head.next;
@@ -48,6 +85,7 @@ export default class LinkedList {
 
     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.value === value) {
         deletedNode = currentNode.next;
@@ -59,12 +97,52 @@ export default class LinkedList {
     return deletedNode;
   }
 
+  deleteByKey(key) {
+    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;
+      }
+      currentNode = currentNode.next;
+    }
+
+    return deletedNode;
+  }
+
+  findByKey(key) {
+    let currentNode = this.head;
+
+    while (currentNode) {
+      if (currentNode.key === key) {
+        return currentNode;
+      }
+      currentNode = currentNode.next;
+    }
+
+    return null;
+  }
+
   toArray() {
     const listArray = [];
     let currentNode = this.head;
 
     while (currentNode) {
-      listArray.push(currentNode.value);
+      listArray.push(currentNode.toString());
       currentNode = currentNode.next;
     }
 

src/data-structures/linked-list/LinkedListNode.js

@@ -1,6 +1,17 @@
 export default class LinkedListNode {
-  constructor(value, next = null) {
+  constructor({ value, next = null, key = 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}`;
   }
 }

src/data-structures/linked-list/README.md

@@ -2,7 +2,7 @@
 
 |Operation                  |Complexity         |
 |---------------------------|-------------------|
-|Indexing                   |O(n)               |
+|Find                       |O(n)               |
 |Insert/delete at beginning |O(1)               |
 |Insert/delete in middle    |O(1) + search time |
 |Insert/delete at end       |O(1) + search time |

src/data-structures/linked-list/__test__/LinkedList.test.js

@@ -9,20 +9,21 @@ describe('LinkedList', () => {
   it('should append node to linked list', () => {
     const linkedList = new LinkedList();
 
-    const node1 = linkedList.append(1);
-    const node2 = linkedList.append(2);
+    const node1 = linkedList.append({ value: 1 });
+    const node2 = linkedList.append({ value: 2, key: 'test' });
 
     expect(node1.value).toBe(1);
     expect(node2.value).toBe(2);
+    expect(node2.key).toBe('test');
 
-    expect(linkedList.toString()).toBe('1,2');
+    expect(linkedList.toString()).toBe('1,test:2');
   });
 
   it('should prepend node to linked list', () => {
     const linkedList = new LinkedList();
 
-    const node1 = linkedList.append(1);
-    const node2 = linkedList.prepend(2);
+    const node1 = linkedList.append({ value: 1 });
+    const node2 = linkedList.prepend({ value: 2 });
 
     expect(node1.value).toBe(1);
     expect(node2.value).toBe(2);
@@ -33,21 +34,53 @@ describe('LinkedList', () => {
   it('should delete node by value from linked list', () => {
     const linkedList = new LinkedList();
 
-    linkedList.append(1);
-    linkedList.append(2);
-    linkedList.append(3);
-    linkedList.append(3);
-    linkedList.append(4);
-    linkedList.append(5);
+    linkedList.append({ value: 1 });
+    linkedList.append({ value: 2 });
+    linkedList.append({ value: 3 });
+    linkedList.append({ value: 3 });
+    linkedList.append({ value: 4 });
+    linkedList.append({ value: 5 });
 
-    const deletedNode = linkedList.delete(3);
+    const deletedNode = linkedList.deleteByValue(3);
     expect(deletedNode.value).toBe(3);
     expect(linkedList.toString()).toBe('1,2,3,4,5');
 
-    linkedList.delete(3);
+    linkedList.deleteByValue(3);
     expect(linkedList.toString()).toBe('1,2,4,5');
 
-    linkedList.delete(1);
+    linkedList.deleteByValue(1);
     expect(linkedList.toString()).toBe('2,4,5');
   });
+
+  it('should delete node by key from linked list', () => {
+    const linkedList = new LinkedList();
+
+    linkedList.append({ value: 1, key: 'test1' });
+    linkedList.append({ value: 2, key: 'test2' });
+    linkedList.append({ value: 3, key: 'test3' });
+
+    const deletedNode = linkedList.deleteByKey('test2');
+    expect(deletedNode.key).toBe('test2');
+    expect(linkedList.toString()).toBe('test1:1,test3:3');
+  });
+
+  it('should append unique nodes', () => {
+    const linkedList = new LinkedList();
+
+    linkedList.appendUnique({ value: 1, key: 'test1' });
+    linkedList.appendUnique({ value: 2, key: 'test2' });
+    linkedList.appendUnique({ value: 3, key: 'test2' });
+
+    expect(linkedList.toString()).toBe('test1:1,test2:3');
+  });
+
+  it('should find node by its key', () => {
+    const linkedList = new LinkedList();
+
+    linkedList.appendUnique({ value: 1, key: 'test1' });
+    linkedList.appendUnique({ value: 2, key: 'test2' });
+    linkedList.appendUnique({ value: 3, key: 'test3' });
+
+    expect(linkedList.findByKey('test3').toString()).toBe('test3:3');
+  });
 });

src/data-structures/linked-list/__test__/LinkedListNode.test.js

@@ -1,9 +1,18 @@
 import LinkedListNode from '../LinkedListNode';
 
 describe('LinkedListNode', () => {
-  it('should create list node with value', () => {
-    const node = new LinkedListNode(1);
+  it('should create list node with kay and value', () => {
+    const node = new LinkedListNode({ value: 1, key: 'test' });
     expect(node.value).toBe(1);
+    expect(node.key).toBe('test');
     expect(node.next).toBeNull();
   });
+
+  it('should convert node to string', () => {
+    const node = new LinkedListNode({ value: 1 });
+    const nodeWithKey = new LinkedListNode({ value: 1, key: 'test' });
+
+    expect(node.toString()).toBe('1');
+    expect(nodeWithKey.toString()).toBe('test:1');
+  });
 });