146. LRU Cache

• Difficulty: Medium

• Topics: Design

• Similar Questions:

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Problem:

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

Solutions:

class LRUCache {
public:
    LRUCache(int capacity) {
        this->capacity = capacity;
    }

    int get(int key) {
        if (keyToNode.count(key) == 0) {
            return -1;
        }

        int val = keyToNode[key]->second;
        cache.splice(cache.begin(), cache, keyToNode[key]);
        return val;
    }

    void put(int key, int value) {
        if (keyToNode.count(key) > 0) {
            cache.splice(cache.begin(), cache, keyToNode[key]);
            keyToNode[key]->second = value;
            return;
        }

        cache.insert(cache.begin(), {key, value});
        keyToNode[key] = cache.begin();

        if (cache.size() > capacity) {
            int key = cache.back().first;
            cache.pop_back();
            keyToNode.erase(key);
        }
    }
private:
    int capacity;
    list<pair<int, int>> cache;
    unordered_map<int, list<pair<int, int>>::iterator> keyToNode;
};

/**
 * Your LRUCache object will be instantiated and called as such:
 * LRUCache* obj = new LRUCache(capacity);
 * int param_1 = obj->get(key);
 * obj->put(key,value);
 */