LFU(Least Frequently Used) C++ 实现

#include 
#include 
#include 
#include 
#include 
using namespace std;
///////////////////////////////////////////////////////////////////////////////////////////////////////////////
// LFU - Least Frequently Used
class FreqNode{
public:
        int freq;
        // We use a SET in lieu of a linked list for storing elements with the same access frequency
    // for simplicitly of implementation.
        // HASH SET structure which holds the keys of such elements that have the same access frequency.
    // Its insertion, lookup and deletion runtime complexity is O(1).
        unordered_set items;
        FreqNode(int f){
                this->freq = f;
        }
};

// LFU - Least Frequently Used Cache Algorithm
// 1 Linked List + 1 Set + 1 Hash
// O(1) for all: Insert, Delete and Lookup
class LFUCache{
public:
        list freqNodes;
        // key   : item (key of element)
        // value : point to its parent freqNode.
        //         Those nodes with the same access frequency share a single parent.
        unordered_map::iterator> bykey;

        FreqNode* createNode(int freq,
                list::iterator next){
                FreqNode* newNode = new FreqNode(freq);
                if(newNode == NULL){
                        return NULL;
                }
                freqNodes.insert(next, newNode);
                return newNode;
        }

        void deleteNode(list::iterator it){
                freqNodes.erase(it);
        }
public:
        LFUCache(){
        }
        ~LFUCache(){
                for(auto it = freqNodes.begin(); it != freqNodes.end(); it++){
                        delete(*it);
                }
        }

        // Access (fetch) an element from the LFU cache,
        // simultaneously incrementing its usage count.
        // Assuming the element(key) already exists beforehand.
        // O(1)
        void access(int key){
                if(bykey.find(key) == bykey.end()){
                        // not exist
                        return ;
                }
                list::iterator nodeIt = bykey[key];
                int freqNow =(*nodeIt)->freq;

                list::iterator nextNodeIt = nodeIt;
                nextNodeIt++;
                if(nextNodeIt == freqNodes.end() || (*nextNodeIt)->freq != (freqNow + 1)){
                        createNode(freqNow+1, nextNodeIt);
                }
                nextNodeIt = nodeIt;
                nextNodeIt++;
                bykey[key] = nextNodeIt;
                (*nextNodeIt)->items.insert(key);

                (*nodeIt)->items.erase(key);
                if((*nodeIt)->items.empty()){
                        deleteNode(nodeIt);
                }
        }

        // Insert a new element.
        // You must make sure no existing element(key) before calling this.
        // O(1)
        void insert(int key){
                if(freqNodes.empty() || (*(freqNodes.begin()))->freq != 1){
                        FreqNode* newNode = new FreqNode(1);
                        freqNodes.insert(freqNodes.begin(), newNode);
                }
                auto nodeIt = freqNodes.begin();
                (*nodeIt)->items.insert(key);
                bykey[key] = nodeIt;
        }

        // Look up an item with key, return its usage count
        // O(1)
        int query(int key){
                if(bykey.find(key) == bykey.end()){
                        // error out
                        return -1;
                }
                return (*bykey[key])->freq;
        }

        // Fetches an item with the least usage count (the least frequently used item)
        // in the cache
        // O(1)
        int getLFUItem(){
                if(freqNodes.empty()){
                        // error out
                        return -1;
                }
                return (*freqNodes.begin())->freq;
        }
};
///////////////////////////////////////////////////////////////////////////////////////////////////////////////
// TEST Program
// Test data from Paper
void generateTestData(LFUCache& lfu){
        vector keys = {'a', 'b', 'c', 'x', 'y', 'z'};
        for(int i = 0; i < 6; i++){
                lfu.insert(keys[i]);
        }
        lfu.access('a');
        for(int i = 1; i < 3; i++){
                lfu.access('z');
        }
        for(int i = 1; i < 5; i++){
                lfu.access('b');
        }
        for(int i = 1; i < 9; i++){
                lfu.access('c');
        }
}
void printFreq(LFUCache& lfu){
        for(auto it = lfu.freqNodes.begin();
                it != lfu.freqNodes.end();
                it++){
                cout<<"Freq = "<<(*it)->freq << ": ";
                for(auto iIt = (*it)->items.begin();
                        iIt != (*it)->items.end();
                        iIt++){
                        cout<<(char)*iIt<<" ";
                }
                cout<