From 4136ce069a48bd2876a9102e46ce6d34cd31c328 Mon Sep 17 00:00:00 2001
From: Nishant Nirmal <nirmalnishant645@gmail.com>
Date: Tue, 24 Mar 2020 10:40:26 +0530
Subject: [PATCH] LRU Cache

---
 0146-LRU-Cache.py | 90 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 90 insertions(+)
 create mode 100644 0146-LRU-Cache.py

diff --git a/0146-LRU-Cache.py b/0146-LRU-Cache.py
new file mode 100644
index 0000000..3f3bca9
--- /dev/null
+++ b/0146-LRU-Cache.py
@@ -0,0 +1,90 @@
+'''
+Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and put.
+
+get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
+put(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.
+
+The cache is initialized with a positive capacity.
+
+Follow up:
+Could you do both operations in O(1) time complexity?
+
+Example:
+
+LRUCache cache = new LRUCache( 2 /* capacity */ );
+
+cache.put(1, 1);
+cache.put(2, 2);
+cache.get(1);       // returns 1
+cache.put(3, 3);    // evicts key 2
+cache.get(2);       // returns -1 (not found)
+cache.put(4, 4);    // evicts key 1
+cache.get(1);       // returns -1 (not found)
+cache.get(3);       // returns 3
+cache.get(4);       // returns 4
+'''
+class Node:
+    
+    def __init__(self, key, value):
+        self.key = key
+        self.value = value
+        self.prev = self
+        self.next = self
+
+class LRUCache:
+
+    def __init__(self, capacity: int):
+        self.d = {}
+        self.capacity = capacity
+        self.head = Node(0, 0)
+        self.tail = Node(1,1)
+        self.head.next = self.tail
+        self.tail.prev = self.head
+        
+
+    def get(self, key: int) -> int:
+        if key in self.d:
+            cur_node = self.d[key]
+            self.remove_from_list(cur_node)
+            self.insert_at_head(cur_node)
+            return cur_node.value
+        else:
+            return -1
+        
+
+    def put(self, key: int, value: int) -> None:
+        if key in self.d:
+            cur_node = self.d[key]
+            self.remove_from_list(cur_node)
+            self.insert_at_head(cur_node)
+            cur_node.value = value
+        else:
+            if len(self.d) >= self.capacity:
+                self.remove_from_tail()
+            cur_node = Node(key, value)
+            self.d[key] = cur_node
+            self.insert_at_head(cur_node)
+            
+    def remove_from_list(self, node):
+        node.prev.next = node.next
+        node.next.prev = node.prev
+        
+    def insert_at_head(self, node):
+        head_next = self.head.next
+        self.head.next = node
+        node.prev = self.head
+        node.next = head_next
+        head_next.prev = node
+        
+    def remove_from_tail(self):
+        if len(self.d) == 0:
+            return
+        tail_node = self.tail.prev
+        del self.d[tail_node.key]
+        self.remove_from_list(tail_node)
+
+
+# Your LRUCache object will be instantiated and called as such:
+# obj = LRUCache(capacity)
+# param_1 = obj.get(key)
+# obj.put(key,value)