146 LRU Cache

Leetcode

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

---

使用链表和 hash 表，达到查找时间复杂度 o(1) ，增加和更新缓存时间复杂度 o(1)

class LRUCache {

    Node head, tail;
    HashMap<Integer, Node> map;
    int capacity, size;

    public LRUCache(int capacity) {
        head=new Node(0,0);
        tail=new Node(0,0);
        head.next=tail;
        tail.pre=head;
        map=new HashMap<>();
        this.capacity=capacity;
        this.size=0;
    }

    public int get(int key) {
        Node node = map.get(key);
        if(node==null){
            return -1;
        }
        node.pre.next=node.next;
        node.next.pre=node.pre;
        insertNextTo(node, head);
        return node.val;
    }

    public void put(int key, int value) {
        Node node = map.get(key);
        if(node!=null){
            node.val=value;
            node.pre.next=node.next;
            node.next.pre=node.pre;
            insertNextTo(node, head);
            return;
        }

        if(size<capacity){
            node = new Node(key,value);
            map.put(key, node);
            size++;
            insertNextTo(node, head);
        } else {
            node = tail.pre;
            map.remove(node.key);
            node.key = key;
            node.val = value;
            tail.pre=node.pre;
            node.pre.next=tail;
            insertNextTo(node, head);
            map.put(key, node);
        }
    }


    private void insertNextTo(Node node, Node nextTo){
        nextTo.next.pre=node;
        node.next=nextTo.next;
        node.pre=nextTo;
        nextTo.next=node;
    }

    private class Node{
        int key,val;
        Node pre,next;
        public Node(int key, int val){
            this.key=key;
            this.val=val;
        }
    }
}

/**
 * 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);
 */