Design a map that allows you to do the following: Maps a string key to a given value. Returns the sum of the values that have a key with a prefix equal to a given string. Implement the MapSum class: MapSum() Initializes the MapSum object. void insert(String key, int val) Inserts the key-val pair into the map. If the key already existed, the original key-value pair will be overridden to the new one. int sum(string prefix) Returns the sum of all the pairs' value whose key starts with the prefix.