146.java

class LRUCache {
    int capacity;
    
    class Node{
        int key,value;
        Node next,prev;
        Node(int k, int v) {
            key = k;
            value = v;
        }
    }
    HashMap<Integer,Node> map;
    Node head,tail;
    public LRUCache(int capacity) {
        this.capacity=capacity;
        map=new HashMap<>(capacity);
        head=new Node(0,0);
        tail=new Node(0,0);
        head.next=tail;
        tail.prev=head;
    }
    public int get(int key) {
        if(!map.containsKey(key)){
            return -1;
        }
        Node node=map.get(key);
        remove(node);
        insertAtHead(node);
        return node.value;
    }
    
    public void put(int key, int value) {
        if(map.containsKey(key)){
            Node node=map.get(key);
            remove(node);
            node.value=value;
            insertAtHead(node);
        }
        else{
            if(map.size()==capacity){
                Node lru=tail.prev;
                remove(lru);
                map.remove(lru.key);
            }
            Node node = new Node(key, value);
            insertAtHead(node);
            map.put(key,node);
        }
    }
    public void remove(Node node){
        node.prev.next=node.next;
        node.next.prev=node.prev;
    }
    public void insertAtHead(Node node){
        head.next.prev=node;
        node.next=head.next;
        head.next=node;
        node.prev=head;
    }
}

/**
 * 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);
 */