## 一、题目

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: `get` and `set`.

`get(key)` - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.

`set(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.

## 三、解题代码

``````public class Solution {
private int capacity;
private HashMap<Integer, Node> map = new HashMap<>();
private Node head = new Node(-1, -1), tail = new Node(-1, -1);

private class Node {
Node prev, next;
int val, key;

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

//         @Override
//         public String toString() {
//             return "(" + key + ", " + val + ") " + "last:"
//                     + (prev == null ? "null" : "node");
//         }
}

public Solution(int capacity) {
this.capacity = capacity;
}

public int get(int key) {
if (!map.containsKey(key)) {
return -1;
}
// remove current
Node currentNode = map.get(key);
currentNode.prev.next = currentNode.next;
currentNode.next.prev = currentNode.prev;

// move current to tail;
moveToTail(currentNode);

return map.get(key).val;
}

public void set(int key, int value) {
if (get(key) != -1) {
map.get(key).val = value;
return;
}
if (map.size() == capacity) {