You are going to implement a hash class that has two functions, put and get. \n```\nvoid put(string key, int exp_time);\nbool get(string key);\n```\nThe put function will save ```key``` with expiration time ```exp_time```.\nThe get function will return true if the key is already put and it\'s has not been expired. Also, let\'s assume that you have a function like ```get_time()``` that return current time.\nThen, the follow up question is how to improve memory complexity. I think this is the difficult part of the problem. For the first part we can use a map of with values as expiration time. However, we need to throw away the keys that have already expired when ```put``` or ```get``` are called to save memory. Also, in calling ```put```, the ```key``` can already be in the map and the new expiration time might be larger than or smaller than the previous one. Any suggestion on the followup question?

You are going to implement a hash class that has two functions, put and get. \n\nvoid put(string key, int exp_time);\nbool get(string key);\n\nThe put function 