LeetCode - LRU Cache

Posted 2020-11-07 IncredibleThings

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.

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

 

 

此题的思路是运用hashmap和double linked nodes

The hash makes the time of get() to be O(1). The list of double linked nodes make the nodes adding/removal operations O(1).

 

class LRUCache {
    class Node{
    int key;
    int value;
    Node pre;
    Node next;
    public Node(int key, int value){
        this.key = key;
        this.value = value;
    }
}
    
    
    int capacity;
    Map<Integer, Node> map = new HashMap<>();
    Node head=null;
    Node end=null;

    public LRUCache(int capacity) {
        this.capacity = capacity;
    }
    
    public int get(int key) {
        if(map.containsKey(key)){
            Node n = map.get(key);
            remove(n);
            setHead(n);
            return n.value;
        }
        return -1;
    }

    public void remove(Node n){
        if(n.pre != null){
            n.pre.next = n.next;
        }
        else{
            head = n.next;
        }
        if(n.next != null){
            n.next.pre = n.pre;
        }
        else{
            end = n.pre;
        } 
    }
    public void setHead(Node n){
        n.next = head;
        n.pre =  null;
        
        if(head != null){
            head.pre = n;
        }
        head = n;
        if(end == null){
            end = head;
        }
    }
    
    public void put(int key, int value) {
        if(map.containsKey(key)){
            Node oldNode = map.get(key);
            oldNode.value = value;
            remove(oldNode);
            setHead(oldNode);
        }
        else{
            Node newNode = new Node(key, value);
            if(map.size() >= capacity){
                map.remove(end.key);
                remove(end);
                setHead(newNode);
            }
            else{
                setHead(newNode);
            }
            map.put(key, newNode);
        }
    }
}

/**
 * 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);
 */