leetcode 460. LFU Cache

最新推荐文章于 2022-09-07 17:53:33 发布

JackZhangNJU

最新推荐文章于 2022-09-07 17:53:33 发布

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分类专栏： leetcode For C++

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本文介绍了一种基于最少使用频率（LFU）原则的缓存数据结构的设计与实现，支持get和put操作，并能够在缓存容量达到上限时，根据LFU原则移除最不常用项。文章还提供了一个C++实现的例子。

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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

不喜欢这样的题，答案是参考网上的答案，就这么做吧

代码如下：

class LFUCache 
{
   int size;
   int minfreq;
   int cap;
   map<int,pair<int,int>> m;//key to pair<value,freq>
   map<int,list<int>::iterator> mIter;//key to list location
   map<int,list<int>> fm;//freq to list
public:
    LFUCache(int capacity)
    {
       cap=capacity;
       size=0;
    }

    int get(int key)
    {
       if(m.count(key)==0) return -1;
       fm[m[key].second].erase(mIter[key]);//删除key在fm原来的位置
       m[key].second++;//频率加一
       fm[m[key].second].push_back(key);//按照频率，放在新的位置上
       mIter[key]=--fm[m[key].second].end();//存储key现在所在的链表例=里的位置
       if(fm[minfreq].size()==0)
           minfreq++;

       return m[key].first;
    }

    void put(int key, int value) 
    {
      if(cap<=0) return;
      int storedValue=get(key);
      if(storedValue!=-1)//若以前已经存在过
      {
          m[key].first=value;
          return;
      }//否则，
      if(size>=cap)//可能要根据LFU删掉一个元素
      {
          m.erase(fm[minfreq].front());
          mIter.erase(fm[minfreq].front());
          fm[minfreq].pop_front();
          size--;
      }
      m[key]={value,1};
      fm[1].push_back(key);
      mIter[key]=--fm[1].end();
      minfreq=1;
      size++;
    }
};