146. LRU Cache

Author: Partho Biswas

README.md

@@ -221,6 +221,7 @@ I have solved quite a number of problems from several topics. See the below tabl
 |20| [160. Intersection of Two Linked Lists](https://tinyurl.com/rz6nrop)| [Python](https://tinyurl.com/wu6rdaw/160_Intersection_of_Tw_Linked_Lists.py)| [Art 1](https://tinyurl.com/urrs7uy), [Art 2](https://tinyurl.com/wkya7ks) | Easy | --- |
 |21| **[138. Copy List with Random Pointer](https://tinyurl.com/uhaw95f)** | [Python](https://tinyurl.com/wu6rdaw/138_Copy_List_with_Random_Pointer.py)| **[Vid 1](https://tinyurl.com/reaqam9), [Art 1](https://tinyurl.com/tnwofvs)** | Medium | **TODO: Check again. Very important. Learned a lot of things** |
 |22| **[430. Flatten a Multilevel Doubly Linked List](https://tinyurl.com/sgpamoh)** | [Python](https://tinyurl.com/wu6rdaw/430_Flatten_a_Multilevel_Doubly_Linked_List.py)| [codinginterviewclass.com](https://tinyurl.com/wtmsrqa), **[Vid 1](https://tinyurl.com/tbp99p2), [Art 1](https://tinyurl.com/un8p7f2)** | Medium | **TODO: Check again. Very important. Learned a lot of things** |
+|23| **[146. LRU Cache](https://tinyurl.com/zu2qbfl)** | [Python](https://tinyurl.com/wu6rdaw/146_LRU_Cache.py)| **[Vid 1](https://tinyurl.com/rtnhhys), [backtobackswe.com](https://tinyurl.com/vbpvjdc), [algoexpert.io](https://tinyurl.com/t2uhpgn)** | Medium | **TODO: Check again. Very important. Learned a lot of things** |
 
 
 </p>

leetcode.com/python/146_LRU_Cache.py

@@ -0,0 +1,104 @@
+# Approach 2: Hashmap + DoubleLinkedList
+class LRUCache(object):
+    def __init__(self, capacity):
+        self.cache = {}
+        self.capacity = capacity or 1
+        self.currentCapacity = 0
+        self.listOfMostRecent = DoublyLinkedList()
+
+
+    # O(1) time | O(1) space
+    def put(self, key, value):
+        if key not in self.cache:
+            if self.currentCapacity == self.capacity:
+                self.evictLeastRecent()
+            else:
+                self.currentCapacity += 1
+            self.cache[key] = DoublyLinkedListNode(key, value)
+        else:
+            self.replaceKey(key, value)
+        self.updateMostRecent(self.cache[key])
+
+
+    # O(1) time | O(1) space
+    def get(self, key):
+        if key not in self.cache:
+            return None
+        self.updateMostRecent(self.cache[key])
+        return self.cache[key].value
+
+
+    # O(1) time | O(1) space
+    def getMostRecentKey(self):
+        return self.listOfMostRecent.head.key
+
+
+    def evictLeastRecent(self):
+        keyToRemove = self.listOfMostRecent.tail.key
+        self.listOfMostRecent.removeTail()
+        del self.cache[keyToRemove]
+
+
+    def updateMostRecent(self, node):
+        self.listOfMostRecent.setToHead(node)
+
+
+    def replaceKey(self, key, value):
+        if key not in self.cache:
+            raise Exception("The provided key isn't in the cache!")
+        self.cache[key].value = value
+
+
+
+
+class DoublyLinkedList:
+    def __init__(self):
+        self.head = None
+        self.tail = None
+
+
+    def setToHead(self, node):
+        if self.head == node:
+            return
+        elif self.head is None:
+            self.head = node
+            self.tail = node
+        elif self.head == self.tail:
+            self.tail.prev = node
+            self.head = node
+            self.head.next = self.tail
+        else:
+            if self.tail == node:
+                self.removeTail()
+            node.removeBindings()
+            self.head.prev = node
+            node.next = self.head
+            self.head = node
+
+
+    def removeTail(self):
+        if self.tail is None:
+            return
+        if self.tail == self.head:
+            self.head = None
+            self.tail = None
+            return
+        self.tail = self.tail.prev
+        self.tail.next = None
+
+
+class DoublyLinkedListNode:
+    def __init__(self, key, value):
+        self.key = key
+        self.value = value
+        self.prev = None
+        self.next = None
+
+
+    def removeBindings(self):
+        if self.prev is not None:
+            self.prev.next = self.next
+        if self.next is not None:
+            self.next.prev = self.prev
+        self.prev = None
+        self.next = None