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 …

LRUCache.java

class LRUCache {
    int capacity;
    Map<Integer, ListNode> map;
    ListNode head, tail;
    private static class ListNode {
        int key, value;
        ListNode prev, next;
        
        ListNode(int key, int value) {
            this.key = key;
            this.value = value;
        }
    }
    
    public LRUCache(int capacity) {
        this.map = new HashMap<>();
        this.capacity = capacity;
    }
    
    public int get(int key) {
        ListNode node = map.get(key);
        if(node == null) return -1;
        
        if(node != head){
            removeFromLinkedList(node);
            insertToFrontOfLinkedList(node);
        }
        
        return node.value;
    }
    
    public void put(int key, int value) {
        removeFromCache(key);
        if(map.size() >= capacity && tail != null)
            removeFromCache(tail.key);
        
        ListNode node = new ListNode(key, value);
        insertToFrontOfLinkedList(node);
        map.put(key, node);
    }
    
    void removeFromCache(int key) {
        removeFromLinkedList(map.get(key));
        map.remove(key);
    }
    
    void removeFromLinkedList(ListNode node) {
        if(node == null)  return;
        
        if(node.prev != null) node.prev.next = node.next;
        if(node.next != null) node.next.prev = node.prev;
        
        if(node == tail) tail = node.prev;
        if(node == head) head = node.next;
    }
    
    void insertToFrontOfLinkedList(ListNode node) {
        if(head == null) {
            head = node;
            tail = node;
        } else {
            head.prev = node;
            node.next = head;           
            head = node;
        }
    }
}