136. Evaluate Reverse Polish Notation

You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation. Evaluate the expression. Return an integer that represents the value of the expression. Note that: - The valid operators are '+', '-', '*', and '/'. - Each operand may be an integer or another expression. - The division between two integers always truncates toward zero. - There will not be any division by zero. - The input represents a valid arithmetic expression in a reverse polish notation. - The answer and all the intermediate calculations can be represented in a 32-bit integer.