146. LRU Cache

1. Description

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

LRUCache(int capacity) Initialize the LRU cache with positive size capacity.
int get(int key) Return the value of the key if the key exists, otherwise return -1.
void put(int key, int value) Update the value of the key if the key exists. Otherwise, add the key-value pair to the cache. If the number of keys exceeds the capacity from this operation, evict the least recently used key.

The functions get and put must each run in O(1) average time complexity.

2. Example

Example 1

Input
[“LRUCache”, “put”, “put”, “get”, “put”, “get”, “put”, “get”, “get”, “get”]
[[2], [1, 1], [2, 2], [1], [3, 3], [2], [4, 4], [1], [3], [4]]
Output
[null, null, null, 1, null, -1, null, -1, 3, 4]

Explanation
LRUCache lRUCache = new LRUCache(2);
lRUCache.put(1, 1); // cache is {1=1}
lRUCache.put(2, 2); // cache is {1=1, 2=2}
lRUCache.get(1); // return 1
lRUCache.put(3, 3); // LRU key was 2, evicts key 2, cache is {1=1, 3=3}
lRUCache.get(2); // returns -1 (not found)
lRUCache.put(4, 4); // LRU key was 1, evicts key 1, cache is {4=4, 3=3}
lRUCache.get(1); // return -1 (not found)
lRUCache.get(3); // return 3
lRUCache.get(4); // return 4

3. Constraints

1 <= capacity <= 3000
0 <= key <= $10^4$
0 <= value <= $10^5$
At most $2 * 10^5$ calls will be made to get and put

4. Solutions

Hash Table

class LRUCache {
public:
    LRUCache(int capacity) : capacity(capacity) {
        dummy = new Node();
        dummy->next = dummy->prev = dummy;
    }

    ~LRUCache() {
        Node *iter = dummy->next;
        while (iter != dummy) {
            Node *next = iter->next;
            delete iter;
            iter = next;
        }

        delete dummy;
    }

    int get(int key) {
        auto iter = key_node.find(key);
        if (iter == key_node.end()) {
            return -1;
        }

        move_to_front(iter->second);
        return iter->second->value;
    }

    void put(int key, int value) {
        auto iter = key_node.find(key);
        if (iter == key_node.end()) {
            auto node = new Node(key, value);
            insert_to_front(node);
            key_node[key] = node;

            if (key_node.size() > capacity) {
                auto node_to_remove = dummy->prev;
                remove_node(node_to_remove);
                key_node.erase(node_to_remove->key);
                delete node_to_remove;
            }
        } else {
            iter->second->value = value;
            move_to_front(iter->second);
        }
    }

private:
    struct Node {
        int key;
        int value;
        Node *prev;
        Node *next;
        Node(int k = 0, int v = 0) : key(k), value(v), prev(nullptr), next(nullptr) {}
    };

    unordered_map<int, Node *> key_node;
    int capacity;
    Node *dummy;

    void remove_node(Node *node) {
        node->prev->next = node->next;
        node->next->prev = node->prev;
    }

    void insert_to_front(Node *node) {
        node->next = dummy->next;
        node->prev = dummy;

        dummy->next->prev = node;
        dummy->next = node;
    }

    void move_to_front(Node *node) {
        remove_node(node);
        insert_to_front(node);
    }
};

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