146. LRU Cache

Design and implement a data structure forLeast Recently Used (LRU) cache. It should support the following operations:getandput.

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 inO(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

Solution)

LRU Cache

class LRUCache {

    class ListNode {
        int key;
        int val;
        ListNode prev, next;
        public ListNode(int key, int val) {
            this.key = key;
            this.val = val;
            this.prev = this.next = null;
        }
    }

    class DoublyLinkedList {
        ListNode head, tail;
        int size = 0;
        public DoublyLinkedList() {
            head = tail = null;
        }

        public void insert(ListNode node) {
            if (head == null) {
                head = tail = node;
                size = 1;
            } else {
                tail.next = node;
                node.prev = tail;
                tail = node;
                size++;
            }
        }

        public void delete(ListNode node) {
            if (head == null) return;
            if (node.prev != null) {
                node.prev.next = node.next;
            } else {
                head = node.next;
            }
            if (node.next != null) {
                node.next.prev = node.prev;
            } else {
                tail = node.prev;
            }
            size--;
        }

        public int size() {
            return this.size;
        }
    }

    DoublyLinkedList list;
    int capacity;
    Map<Integer, ListNode> map;
    public LRUCache(int capacity) {
        this.capacity = capacity;
        list = new DoublyLinkedList();
        map = new HashMap<>();
    }

    public int get(int key) {
        // if the value is hit, update the list
        if (map.get(key) != null) {
            list.delete(map.get(key));
            ListNode node = new ListNode(key, map.get(key).val);
            map.remove(key);
            map.put(key, node);
            list.insert(node);
            return node.val;
        }
        return -1;
    }

    public void put(int key, int value) {
        if (map.get(key) != null) {
            list.delete(map.get(key));
            map.remove(key);
        } else if (list.size() == capacity) {
            // remove head
            map.remove(list.head.key);
            list.delete(list.head);
        }
        ListNode node = new ListNode(key, value);
        map.put(key, node);
        list.insert(node);
    }
}

/**
 * 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);
 */