146. LRU Cache

[fockspaces]

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.

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

Constraints:

1 <= capacity <= 3000 0 <= key <= 104 0 <= value <= 105 At most 2 * 105 calls will be made to get and put.

[fockspaces]

這題也是純欣賞題... 一般狀況下很難想到

1. cache (map) 存 <key, node> pair
2. 設 begin, end 兩個 node，一開始先互指
3. cache.size() 作為當前存過的 size
4. 將 insert, remove node 行為分開寫 邏輯會更清楚

class LRUCache {
public:
    class Node {
    public: 
        int key, val;
        Node *prev, *next;
        Node(int key = -1, int val = -1) {
            this->key = key;
            this->val = val;
        }
    };
    Node* begin = new Node();
    Node* end = new Node();
    unordered_map<int, Node*> cache;
    int capacity;

    LRUCache(int capacity) {
        this->capacity = capacity;
        begin->next = end; end->prev = begin;  
    }

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

    void insert(Node* node) {
        Node* recentNode = end->prev;
        recentNode->next = node; end->prev = node;
        node->prev = recentNode; node->next = end;
    }

    int get(int key) {
        if(cache.count(key)) {
            remove(cache[key]);
            insert(cache[key]);
            return cache[key]->val;
        }
        return -1;
    }

    void put(int key, int value) {
        if(cache.count(key)) remove(cache[key]);
        Node* node = new Node(key, value);
        insert(node);
        cache[key] = node;
        if(cache.size() > capacity) {
            Node* lastNode = begin->next;
            remove(lastNode);
            cache.erase(lastNode->key);
        }
    }
};

/**
 * 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);
 */

[fockspaces]

重寫一遍犯的錯誤：

1. 宣告 this-> 錯邊
2. 忘記實作 insert, remove method

class LRUCache {
public:
    class Node {
    public:
        int key, val;
        Node *prev, *next;
        Node(int key = -1, int val = -1) {
            this->key = key; this->val = val;
        }
    };
    int capacity;
    Node *begin = new Node(); Node* end = new Node();
    unordered_map<int, Node*> cache;
    LRUCache(int capacity) {
        this->capacity = capacity;
        begin->next = end; end->prev = begin;
    }

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

    void insert(Node* node) {
        Node* recentNode = end->prev;
        recentNode->next = node; end->prev = node;
        node->prev = recentNode; node->next = end;
    }

    int get(int key) {
        if(cache.count(key)) {
            remove(cache[key]);
            insert(cache[key]);
            return cache[key]->val;
        }
        return -1;
    }

    void put(int key, int value) {
        if(cache.count(key)) remove(cache[key]);
        Node* newNode = new Node(key, value);
        insert(newNode);
        cache[key] = newNode;
        if(cache.size() > capacity) {
            Node* deleteNode = begin->next;
            remove(deleteNode);
            cache.erase(deleteNode->key);
        }
    }
};

/**
 * 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);
 */