Merge pull request knaxus#32 from knaxus/DoublyLinkList

doubly link list

Author: ashokdey

README.md

@@ -34,9 +34,9 @@ Collection of interview questions with Unit Tests. Problems includes Data Struct
 - [Queue](src/_DataStructures_/Queue)
   - [Weave](src/_DataStructures_/Queue/weave)
 
+- [Doubly Linked List](src/_DataStructures_/DoublyLinkedList)
 - [Suffix Tree](src/_DataStructures_/SuffixTree)
 
-
 ### Logical Problems
 
 - [Anagrams](src/_Problems_/anagrams)
@@ -63,6 +63,11 @@ Collection of interview questions with Unit Tests. Problems includes Data Struct
 
 - [Binary Search](src/_Searching_/BinarySearch)
 
+### Algorithms
+
+- [LRU Cache](src/_Algorithms_/lru-cache)
+
+
 ### Path Finder
 
 - [A\*](src/PathFinder/AStart)

src/_Algorithms_/lru-cache/index.js

@@ -0,0 +1,71 @@
+/*
+Least recently used (LRU) - cache implementation
+
+get(key) – Get the value (will always be positive) of the key if the key exists in the cache, otherwise return false.
+Complexity: O(1)
+
+set(key, value) – Set or insert the value if the key is not already present. When the cache reached its capacity, it should invalidate the least recently used item before inserting a new item.
+Complexity: O(1)
+*/
+
+const DoublyLinkedList = require('../../_DataStructures_/DoublyLinkedList/index');
+
+class LRUCache {
+  constructor(n) {
+    this.size = n;
+    this.map = new Map();
+    this.list = new DoublyLinkedList();
+  }
+
+  // this method will work in O(1)
+  set(key, value) {
+    const data = {
+      key,
+      value,
+    };
+    if (!this.map.has(key)) {
+      this.list.addAtBeginning(data);
+      this.map.set(key, this.list.head.next);
+
+      if (this.list.length() > this.size) {
+        const lastNode = this.list.tail.previous.data;
+        this.map.delete(lastNode.key);
+        this.list.removeAtEnd();
+      }
+    } else {
+      this.list.remove(this.map.get(key));
+      this.list.addAtBeginning(data);
+      this.map.set(key, this.list.head.next);
+    }
+  }
+
+  // this method will work in O(1)
+  get(key) {
+    if (this.map.has(key)) {
+      const node = this.map.get(key);
+      const { value } = node.data;
+      this.list.remove(node);
+      this.list.addAtBeginning({
+        key,
+        value,
+      });
+      this.map.set(key, this.list.head.next);
+    }
+    return false;
+  }
+}
+
+// const lru = new LRUCache(3);
+// lru.set(1, 1);
+// lru.set(2, 2);
+// lru.set(3, 3);
+// lru.set(4, 4);
+// lru.set(5, 5);
+// lru.set(2, 2);
+// lru.get(5, 5);
+// lru.list.display();
+
+
+module.exports = {
+  LRUCache,
+};

src/_DataStructures_/DoublyLinkedList/index.js

@@ -0,0 +1,65 @@
+/* eslint-disable class-methods-use-this */
+class Node {
+  constructor(data, previous, next) {
+    this.data = data;
+    this.previous = previous;
+    this.next = next;
+  }
+}
+
+class DoublyLinkedList {
+  constructor() {
+    // head -> tail
+    // head <- tail
+    this.head = new Node(null, null, null);
+    this.tail = new Node(null, null, null);
+    this.head.next = this.tail; // head next point to tail
+    this.tail.previous = this.head; // tail previous point to head
+    this.size = 0;
+  }
+
+  addAtBeginning(value) {
+    const newNode = new Node(value, this.head, this.head.next);
+    this.head.next.previous = newNode;
+    this.head.next = newNode;
+    this.size += 1;
+  }
+
+  addAtEnd(value) {
+    const newNode = new Node(value, this.tail.previous, this.tail);
+    this.tail.previous.next = newNode;
+    this.tail.previous = newNode;
+    this.size += 1;
+  }
+
+  removeAtBeginning() {
+    this.remove(this.head.next);
+    this.size -= 1;
+  }
+
+  removeAtEnd() {
+    this.remove(this.tail.previous);
+    this.size -= 1;
+  }
+
+  remove(node) {
+    const previousNode = node.previous;
+    const nextNode = node.next;
+    previousNode.next = nextNode;
+    nextNode.previous = previousNode;
+  }
+
+  length() {
+    return this.size;
+  }
+
+  display() {
+    let address = this.head.next;
+    while (address !== this.tail) {
+      console.log(address.data);
+      address = address.next;
+    }
+  }
+}
+
+module.exports = DoublyLinkedList;