Implement a MapSum class with insert, and sum methods.
For the method insert, you'll be given a pair of (string, integer). The string
represents the key and the integer represents the value. If the key already existed, then
the original key-value pair will be overridden to the new one.
For the method sum, you'll be given a string representing the prefix, and you
need to return the sum of all the pairs' value whose key starts with the prefix.
Example 1:
Input: insert("apple", 3), Output: Null
Input: sum("ap"), Output: 3
Input: insert("app", 2), Output: Null
Input: sum("ap"), Output: 5