Design a data structure that follows the constraints of a Least Recently Used (LRU) cache.

Implement the LRUCache class:

  • LRUCache(int capacity) Initialize the LRU cache with positive size capacity.
  • int get(int key) Return the value of the key if the key exists, otherwise return -1.
  • void put(int key, int value) Update the value of the key if the key exists. Otherwise, add the key-value pair to the cache. If the number of keys exceeds the capacity from this operation, evict the least recently used key.

The functions get and put must each run in O(1) average time complexity.

 

Example 1:

Input
["LRUCache", "put", "put", "get", "put", "get", "put", "get", "get", "get"]
[[2], [1, 1], [2, 2], [1], [3, 3], [2], [4, 4], [1], [3], [4]]
Output
[null, null, null, 1, null, -1, null, -1, 3, 4]

Explanation
LRUCache lRUCache = new LRUCache(2);
lRUCache.put(1, 1); // cache is {1=1}
lRUCache.put(2, 2); // cache is {1=1, 2=2}
lRUCache.get(1);    // return 1
lRUCache.put(3, 3); // LRU key was 2, evicts key 2, cache is {1=1, 3=3}
lRUCache.get(2);    // returns -1 (not found)
lRUCache.put(4, 4); // LRU key was 1, evicts key 1, cache is {4=4, 3=3}
lRUCache.get(1);    // return -1 (not found)
lRUCache.get(3);    // return 3
lRUCache.get(4);    // return 4

 

Constraints:

  • 1 <= capacity <= 3000
  • 0 <= key <= 104
  • 0 <= value <= 105
  • At most 2 * 105 calls will be made to get and put.

class LRUCache {

class Node {
int key;
int value;
Node prev;
Node next;

Node(int k, int v) {
key = k;
value = v;
}
}
private Map<Integer, Node> map = new HashMap<>();
private Node head;
private Node last;
private int capacity;

public LRUCache(final int capacity) {
this.capacity = capacity;
head = new Node(0, 0);
last = new Node(0, 0);
head.next = last;
last.prev = head;
}

public int get(int key) {
Node node = map.get(key);
if (node == null) {
return -1;
}
remove(node);
insert(node);
return node.value;
}

    public void put(int key, int value) {
if (map.containsKey(key)) {
remove(map.get(key));
}
if (map.size() == capacity) {
remove(last.prev);
}
insert(new Node(key, value));
}
    
private void insert(Node node) {
map.put(node.key, node);
node.next = head.next;
head.next.prev = node;
node.prev = head;
head.next = node;
}

private void remove(Node node) {
map.remove(node.key);
node.prev.next = node.next;
node.next.prev = node.prev;
}

}





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