124. calculate arithmetic expression

In 119. create a tokenizer, you are able to extract the tokens from a string with invalid spaces.

Now please calculate() the result of the string. You can use the tokenizer you wrote before.

calculate('1 * (20 -   300      ) ')
// -280

calculate('     1/0 ')
// Infinity

1. the input expression is syntactically valid, containing non-negative integers, +, -, *, /, (, ) and spaces
2. Don't use eval()