Two small refactorings
Test: 1000
TPP: constant -> constant+ (adding another pair into map), similar as 100
Test: 2000
TPP: constant -> constant+ (another pair into map)
Refactoring: after the transformation above, I did a similar refactoring for "thousand" as "hundred".
Next, I did another refactoring to remove the duplication between "thousand" and "hundred".
Refactoring: the duplication of "thousand" and "hundred" code has been removed by a new counting unit map and a for loop