codewars kata - https://www.codewars.com/kata/52a78825cdfc2cfc87000005/java

Instructions Given a mathematical expression as a string you must return the result as a number.

Numbers Number may be both whole numbers and/or decimal numbers. The same goes for the returned result.

Operators You need to support the following mathematical operators:

Multiplication * Division / (as floating point division) Addition + Subtraction - Operators are always evaluated from left-to-right, and * and / must be evaluated before + and -.

Parentheses You need to support multiple levels of nested parentheses, ex. (2 / (2 + 3.33) * 4) - -6

Whitespace There may or may not be whitespace between numbers and operators.

An addition to this rule is that the minus sign (-) used for negating numbers and parentheses will never be separated by whitespace. I.e all of the following are valid expressions.

1-1 // 0 1 -1 // 0 1- 1 // 0 1 - 1 // 0 1- -1 // 2 1 - -1 // 2 1--1 // 2

6 + -(4) // 2 6 + -( -4) // 10 And the following are invalid expressions

1 - - 1 // Invalid 1- - 1 // Invalid 6 + - (4) // Invalid 6 + -(- 4) // Invalid Validation You do not need to worry about validation - you will only receive valid mathematical expressions following the above rules.

Restricted APIs NOTE: To keep up the difficulty of the kata, use of some classes and functions is disallowed. Their names cannot appear in the solution file, even in comments and variable names.