Question
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
Solution
TODO
Code
class LRUCache {
// double linked list
class Node {
// we need the key here
// because when we remove the node from a list we
// need to remove it from the hash table as well
int key, value;
Node prev;
Node next;
public Node(int key, int value) {
this.key = key;
this.value = value;
}
}
Node head;
Node tail;
int capacity;
Map<Integer, Node> map;
public LRUCache(int capacity) {
this.capacity = capacity;
map = new HashMap<Integer, Node>();
// dummy nodes could avoid a lot of trouble
head = new Node(0, 0);
tail = new Node(0, 0);
head.next = tail;
tail.prev = head;
tail.next = null;
head.prev = null;
}
public int get(int key) {
if (!map.containsKey(key)) {
return -1;
}
Node node = map.get(key);
updateToHead(node);
return node.value;
}
public void put(int key, int value) {
if (!map.containsKey(key)) {
Node node = new Node(key, value);
map.put(key, node);
if (map.keySet().size() > capacity) {
Node lastNode = tail.prev;
map.remove(lastNode.key);
deleteNode(lastNode);
}
addToHead(node);
} else {
map.get(key).value = value;
updateToHead(map.get(key));
}
}
private void updateToHead(Node node) {
deleteNode(node);
addToHead(node);
}
private void deleteNode(Node node) {
node.prev.next = node.next;
node.next.prev = node.prev;
}
private void addToHead(Node node) {
Node firstNode = head.next;
head.next = node;
node.next = firstNode;
node.prev = head;
firstNode.prev = 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);
*/
Performance
TODO