Created LRU_Cache.cpp

Design a data structure that follows the constraints of a Least Recently Used (LRU) cache.

Author: bilwa496

146. LRU Cache/LRU_Cache.cpp

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+// Design a data structure that follows the constraints of a Least Recently Used (LRU) cache.
+
+struct Node {
+  int key;
+  int value;
+  Node(int key, int value) : key(key), value(value) {}
+};
+
+
+class LRUCache {
+ public:
+  LRUCache(int capacity) : capacity(capacity) {}
+
+  int get(int key) {
+    if (!keyToIterator.count(key))
+      return -1;
+
+    const auto& it = keyToIterator[key];
+    // move it to the front
+    cache.splice(begin(cache), cache, it);
+    return it->value;
+  }
+
+  void put(int key, int value) {
+    // no capacity issue, just update the value
+    if (keyToIterator.count(key)) {
+      const auto& it = keyToIterator[key];
+      // move it to the front
+      cache.splice(begin(cache), cache, it);
+      it->value = value;
+      return;
+    }
+
+    // check the capacity
+    if (cache.size() == capacity) {
+      const auto& lastNode = cache.back();
+      // that's why we store `key` in `Node`
+      keyToIterator.erase(lastNode.key);
+      cache.pop_back();
+    }
+
+    cache.emplace_front(key, value);
+    keyToIterator[key] = begin(cache);
+  }
+
+ private:
+  const int capacity;
+  list<Node> cache;
+  unordered_map<int, list<Node>::iterator> keyToIterator;
+};
+