[ymeng] calculation: calculate expression with tree hierarchy
[ymeng] calculation: refactor to extract common methods
[ymeng] calculation: add tests for simple subtraction, multiplication & division
[ymeng] tree calculation: move to new package
[ymeng] stack-calculator: transform simple infix expression
[ymeng] stack-calculator: transform infix expression that is combined with addition and multiplication
[ymeng] stack-calculator: transform infix expression that contains brackets
[ymeng] stack-calculator: transform infix expression that contains multi operands in brackets
[ymeng] stack-calculator: transform infix expression that two lower priority operators surround one higher priority operator
[ymeng] stack-calculator: add a comprehensive test
[ymeng] stack-calculator: refactor
[ymeng] stack-calculator: implement a calculator for postfix expression
[ymeng] stack-calculator: should pop operands in correct order
[ymeng] stack-calculator: complete
[ymeng] stack-calculator: encapsulate calculation with command strategy
[ymeng] stack-calculator: extract const