Change the name of Parser's parseSolve() to solve()

Signed-off-by: Hyeoncheol Lee <1982bom@gmail.com>
Add IntegerParser class

Signed-off-by: Hyeoncheol Lee <1982bom@gmail.com>
Add how to minimize cost of joining two strings

Signed-off-by: Hyeoncheol Lee <1982bom@gmail.com>