LRU Cache.cpp

/*
LRU Cache
=========

Design a data structure that follows the constraints of a Least Recently Used (LRU) cache.

Implement the LRUCache class:
LRUCache(int capacity) Initialize the LRU cache with positive size capacity.
int get(int key) Return the value of the key if the key exists, otherwise return -1.
void put(int key, int value) Update the value of the key if the key exists. Otherwise, add the key-value pair to the cache. If the number of keys exceeds the capacity from this operation, evict the least recently used key.

Follow up:
Could you do get and put in O(1) time complexity?

Example 1:
Input
["LRUCache", "put", "put", "get", "put", "get", "put", "get", "get", "get"]
[[2], [1, 1], [2, 2], [1], [3, 3], [2], [4, 4], [1], [3], [4]]
Output
[null, null, null, 1, null, -1, null, -1, 3, 4]

Explanation
LRUCache lRUCache = new LRUCache(2);
lRUCache.put(1, 1); // cache is {1=1}
lRUCache.put(2, 2); // cache is {1=1, 2=2}
lRUCache.get(1);    // return 1
lRUCache.put(3, 3); // LRU key was 2, evicts key 2, cache is {1=1, 3=3}
lRUCache.get(2);    // returns -1 (not found)
lRUCache.put(4, 4); // LRU key was 1, evicts key 1, cache is {4=4, 3=3}
lRUCache.get(1);    // return -1 (not found)
lRUCache.get(3);    // return 3
lRUCache.get(4);    // return 4

Constraints:
1 <= capacity <= 3000
0 <= key <= 3000
0 <= value <= 104
At most 3 * 104 calls will be made to get and put.
*/

class LRUCache
{
public:
  unordered_map<int, pair<int, list<int>::iterator>> Cache;
  list<int> order;
  int size;

  LRUCache(int capacity)
  {
    size = capacity;
  }

  int get(int key)
  {
    if (Cache.find(key) == Cache.end())
      return -1;
    int ans = Cache[key].first;
    auto itr = Cache[key].second;
    order.erase(itr);
    order.push_front(key);
    Cache[key].second = order.begin();
    return ans;
  }

  void put(int key, int value)
  {
    if (Cache.find(key) == Cache.end())
    {
      if (order.size() == size)
      {
        auto oldest = order.back();
        Cache.erase(oldest);
        order.pop_back();
      }
      order.push_front(key);
      Cache[key] = {value, order.begin()};
    }
    else
    {
      order.erase(Cache[key].second);
      order.push_front(key);
      Cache[key] = {value, order.begin()};
    }
  }
};

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