460. LFU Cache

460. LFU Cache

Hard

Design and implement a data structure for Least Frequently Used (LFU) 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 reaches its capacity, it should invalidate the least frequently used item before inserting a new item. For the purpose of this problem, when there is a tie (i.e., two or more keys that have the same frequency), the least recently used key would be evicted.

Follow up:
Could you do both operations in O(1) time complexity?

Example:

LFUCache cache = new LFUCache( 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.get(3);       // returns 3.
cache.put(4, 4);    // evicts key 1.
cache.get(1);       // returns -1 (not found)
cache.get(3);       // returns 3
cache.get(4);       // returns 4

--------------------
1个hashmap存key - value
1个存key - frequency
1个存 frequency - keys

用min来存最小的frequency
LinkedHashSet里面是hashset + doubly linked list的实现方式，所以是有序的

class LFUCache {

    private Map<Integer, Integer> map;
    private Map<Integer, Integer> keyToFrq;
    private Map<Integer, LinkedHashSet<Integer>> frqToKeys;
    private int cap;
    private int min;
    
    public LFUCache(int capacity) {
        min = -1;
        cap = capacity;
        map = new HashMap<>();
        keyToFrq = new HashMap<>();
        frqToKeys = new HashMap<>();
    }
    
    public int get(int key) {

        if (!map.containsKey(key)) return-1;
        int frq = keyToFrq.get(key);
        frqToKeys.get(frq).remove(key);
        if (min == frq && frqToKeys.get(frq).isEmpty()) min++;
        
        frq++;
        if (!frqToKeys.containsKey(frq)) frqToKeys.put(frq, new LinkedHashSet<Integer>());
        frqToKeys.get(frq).add(key);
        keyToFrq.put(key, keyToFrq.get(key) + 1);
        
        return map.get(key);
    }
    
    public void put(int key, int value) {
        if (cap <= 0) return;
        if (map.containsKey(key)) {
            map.put(key, value);
            get(key);
            return;
        }
        
        if (map.size() >= cap) {
            
            int evit = frqToKeys.get(min).iterator().next();
            frqToKeys.get(min).remove(evit);
            map.remove(evit);
            keyToFrq.remove(evit);
        }
        
        map.put(key, value);
        min = 1;
        keyToFrq.put(key, min);
        if (!frqToKeys.containsKey(min)) frqToKeys.put(min, new LinkedHashSet<Integer>());
        frqToKeys.get(min).add(key);
    }
}

/**
 * Your LFUCache object will be instantiated and called as such:
 * LFUCache obj = new LFUCache(capacity);
 * int param_1 = obj.get(key);
 * obj.put(key,value);
 */