Function $1$: postfixEval$($arr$)$

Write a function named postfixEval such that given a postfix expression, evaluate it and return the result.

Assumptions:

the operands are floats

the possible operators are $+,-,**\frac{,}{?}$

the input expression is always a valid and none$-$empty postfix expression

Example: When input is $["2","4","7","*","-"],$ which is $2-4**7$ using infix notation, your function should $-26.$

float postfixEval$($const std::vector& tokens$)\{$

$//$ TODO: Student to implement

return $0\mathrm{.}0$f; $//$ Placeholder return

$\}$