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146. LRU Cache

146. LRU Cache

作者: Jeanz | 来源:发表于2018-01-03 09:36 被阅读0次

    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
    

    一刷
    题解:
    用double linkedlist, 最新加入/访问过的插到head, remove时从尾部
    并且用map快速定位node

    class LRUCache {
        
        final Node head = new Node(0, 0);
        final Node tail = new Node(0, 0);
        final Map<Integer, Node> map;
        final int capacity;
        
        public LRUCache(int capacity) {
            this.capacity = capacity;
            map = new HashMap(capacity);
            head.next = tail;
            tail.prev = head;
        }
        
        public int get(int key) {
          int res = -1;
          if(map.containsKey(key)){
            Node n = map.get(key);
            remove(n);
            insertToHead(n);
            res = n.value;
          }
          return res;   
        }
        
        public void put(int key, int value) {
          if(map.containsKey(key)){
            Node n = map.get(key);
            remove(n);
            n.value = value;
            insertToHead(n);
          } else {
            
            if(map.size() == capacity){
               map.remove(tail.prev.key); 
               remove(tail.prev);
            } 
            
            Node n = new Node(key, value);
            insertToHead(n);
            map.put(key, n);
          }  
        }
        
        private void remove(Node n){
          n.prev.next = n.next;
          n.next.prev = n.prev;
        }
        
        private void insertToHead(Node n){
          Node headNext = head.next;
          head.next = n;
          headNext.prev = n;
          n.prev = head;
          n.next = headNext;
        }
        
        class Node{
          Node prev, next;
          int key, value;
          Node(int k, int v){
            key = k;
            value = v;
          }
        }
    }
    
    /**
     * 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);
     */
    

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