146. LRU Cache

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146. LRU Cache

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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.

难度 : Medium

思路

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class LRUCache {

    class Node{

        int key;

        int value;

        Node prev;

        Node next;

        public Node(int key, int value) {

            this.key = key;

            this.value = value;

        }

        public Node(){

            

        }

    }

    

    Map<Integer,Node> map;

    Node head;

    Node tail;

    int capacity;

    

    public LRUCache(int capacity) {

        this.map = new HashMap<>();

        this.capacity = capacity;

        this.head = new Node();

        this.tail = new Node();

        head.next = tail;

        tail.prev = head;

    }

    

    public int get(int key) {

        if (map.containsKey(key)) {

            Node node = map.get(key);

            touch(node);

            return node.value;        

        } else {

            return -1;

        }

    }

    

    

    private void touch(Node node) {

        remove(node);

        addToHead(node);

    }

    

    

    private void remove(Node node) {

        Node prev = node.prev;

        Node next = node.next;

        prev.next = next;

        next.prev = prev;

    }

    

    private void addToHead(Node node) {

        Node next = head.next;

        node.next = next;

        next.prev = node;

        head.next = node;

        node.prev = head;

    }

    

    public void put(int key, int value) {

        if (map.containsKey(key)) {

            Node node = map.get(key);

            node.value = value;

            touch(node);            

            map.put(key, node);

        } else {

            if (capacity == map.size()) {

                //remove last

                Node last = tail.prev;

                map.remove(last.key);

                remove(last);

            }

            Node node = new Node(key, value);            

            addToHead(node);

            map.put(key, node);

        }

    }

}