Design an LRU cache with all the operations including getting the least recently used item to be O(1)

#include <iostream>

#include <vector>
#include <hash_map>
using namespace std;
template <class K, class T>
struct LRUCacheEntry
{
   K key;
   T data;
   LRUCacheEntry * prev;
   LRUCacheEntry * next;
};
template <class K, class T>
class LRUCache
{
public:
  LRUCache(size_t sz) : head(NULL), tail(NULL){
    entries = new LRUCacheEntry<K, T>[sz];
    for (int i = 0; i < sz; i++)
      freeEntries.push_back(entries + i);
  }
  
  ~LRUCache(){  delete[] entries; }
  bool containsKey(K key) { return _m.find(key) != _m.end(); }
  void put(K key, T data){
    LRUCacheEntry<K, T> * entry;
    
    if (_m.find(key) != _m.end()) return;
    if (freeEntries.size() > 0){
      entry = freeEntries.back();
      freeEntries.pop_back();
    }
    else{
      // no free entries
      entry = tail;
      if (tail->prev != NULL) tail->prev->next = NULL;
      _m.erase(entry->key);
    }
     
    _m[key] = entry;
    entry->key = key;
    entry->data = data;
    entry->next = head;
    entry->prev = NULL;
    if (head != NULL){
      head->prev = entry;
    }
     
    head = entry;
  }
  // we assumes that containsKey is always called before this method
  T get(K key) { 
    LRUCacheEntry<K, T> * entry = _m[key];
  
   if (entry != head){
      if (entry->next != NULL)
      entry->next->prev = entry->prev;
      entry->prev->next = entry->next;
      if (entry == tail) tail = entry->prev;
      entry->next = head;
      entry->prev = NULL;
      head = entry;
    }
    return entry->data;
  }
  void print(){
    LRUCacheEntry<K, T> * entry = head;
    while (entry != NULL){
      cout << entry->data << " ";
      entry = entry->next;
    }
    cout << endl;
  }
private:
  hash_map<K, LRUCacheEntry<K, T> *> _m;
  LRUCacheEntry<K, T> * head, *tail, *entries;
  vector<LRUCacheEntry<K, T> *> freeEntries;
};
int main()
{
  LRUCache<int, int>  cache(3);
  cache.put(3, 3);
  cache.put(4, 4);
  cache.put(5, 5);
   
  cache.print();
  cache.get(3);
  cache.print();
  return 0;
}