implement O(1) LRU Cache with eviction policy

Author: RomanSanaye

Sprint-2/implement_lru_cache/lru_cache.py

@@ -0,0 +1,74 @@
+class Node:
+    # Create a building block for the cache
+    def __init__(self, key, value):
+        self.key = key
+        self.value = value
+        self.next = None
+        self.previous = None
+
+
+class LruCache:
+    # Initializes the cache with a maximum capacity
+    def __init__(self, limit):
+        if limit <= 0:
+            raise ValueError("Cache limit must be greater than zero.")
+        self.limit = limit
+        self.cache = {}  # Maps key -> Node for O(1) lookups
+        self.head = None  # Most recently used
+        self.tail = None  # Least recently used
+
+    # Looks up a value and marks it as recently used
+    def get(self, key):
+        if key not in self.cache:
+            return None
+
+        node = self.cache[key]
+        self._move_to_head(node)
+        return node.value
+
+    # Add or update an item, evicting the oldest if full
+    def set(self, key, value):
+        if key in self.cache:
+            node = self.cache[key]
+            node.value = value
+            self._move_to_head(node)
+            return
+
+        if len(self.cache) >= self.limit:
+            if self.tail is not None:
+                del self.cache[self.tail.key]
+                
+                if self.head == self.tail:
+                    self.head = None
+                    self.tail = None
+                else:
+                    self.tail = self.tail.previous
+                    self.tail.next = None
+
+        new_node = Node(key, value)
+        self.cache[key] = new_node
+
+        if self.head is None:
+            self.head = new_node
+            self.tail = new_node
+        else:
+            new_node.next = self.head
+            self.head.previous = new_node
+            self.head = new_node
+
+    # Internal helper to reposition a used node to the front
+    def _move_to_head(self, node):
+        if self.head == node:
+            return
+
+        if node == self.tail:
+            self.tail = node.previous
+            self.tail.next = None
+        else:
+            node.previous.next = node.next
+            node.next.previous = node.previous
+
+        node.next = self.head
+        self.head.previous = node
+        node.previous = None
+        self.head = node