Leetcode

LFU Cache

  • Time:O(1)
  • Space:O(n)

C++

struct Node {
  int key;
  int value;
  int freq;
  list<int>::const_iterator it;
};

class LFUCache {
 public:
  LFUCache(int capacity) : capacity(capacity), minFreq(0) {}

  int get(int key) {
    if (!keyToNode.count(key))
      return -1;

    Node& node = keyToNode[key];
    touch(node);
    return node.value;
  }

  void put(int key, int value) {
    if (capacity == 0)
      return;
    if (keyToNode.count(key)) {
      Node& node = keyToNode[key];
      node.value = value;
      touch(node);
      return;
    }

    if (keyToNode.size() == capacity) {
      // evict LRU key from the minFreq list
      const int keyToEvict = freqToList[minFreq].back();
      freqToList[minFreq].pop_back();
      keyToNode.erase(keyToEvict);
    }

    minFreq = 1;
    freqToList[1].push_front(key);
    keyToNode[key] = {key, value, 1, cbegin(freqToList[1])};
  }

 private:
  int capacity;
  int minFreq;
  unordered_map<int, Node> keyToNode;
  unordered_map<int, list<int>> freqToList;

  void touch(Node& node) {
    // update the node's frequency
    const int prevFreq = node.freq;
    const int newFreq = ++node.freq;

    // remove the iterator from prevFreq's list
    freqToList[prevFreq].erase(node.it);
    if (freqToList[prevFreq].empty()) {
      freqToList.erase(prevFreq);
      // update minFreq if needed
      if (prevFreq == minFreq)
        ++minFreq;
    }

    // insert the key to the front of newFreq's list
    freqToList[newFreq].push_front(node.key);
    node.it = cbegin(freqToList[newFreq]);
  }
};

JAVA

class LFUCache {
  public LFUCache(int capacity) {
    this.capacity = capacity;
  }

  public int get(int key) {
    if (!keyToVal.containsKey(key))
      return -1;

    final int freq = keyToFreq.get(key);
    freqToLRUKeys.get(freq).remove(key);
    if (freq == minFreq && freqToLRUKeys.get(freq).isEmpty()) {
      freqToLRUKeys.remove(freq);
      ++minFreq;
    }

    // increase key's freq by 1
    // add this key to next freq's list
    putFreq(key, freq + 1);
    return keyToVal.get(key);
  }

  public void put(int key, int value) {
    if (capacity == 0)
      return;
    if (keyToVal.containsKey(key)) {
      keyToVal.put(key, value);
      get(key); // update key's count
      return;
    }

    if (keyToVal.size() == capacity) {
      // evict LRU key from the minFreq list
      final int keyToEvict = freqToLRUKeys.get(minFreq).iterator().next();
      freqToLRUKeys.get(minFreq).remove(keyToEvict);
      keyToVal.remove(keyToEvict);
    }

    minFreq = 1;
    putFreq(key, minFreq);    // add new key and freq
    keyToVal.put(key, value); // add new key and value
  }

  private int capacity;
  private int minFreq = 0;
  private Map<Integer, Integer> keyToVal = new HashMap<>();
  private Map<Integer, Integer> keyToFreq = new HashMap<>();
  private Map<Integer, LinkedHashSet<Integer>> freqToLRUKeys = new HashMap<>();

  private void putFreq(int key, int freq) {
    keyToFreq.put(key, freq);
    freqToLRUKeys.putIfAbsent(freq, new LinkedHashSet<>());
    freqToLRUKeys.get(freq).add(key);
  }
}