You are given an array of strings `tokens` that represents an arithmetic expression in **Reverse Polish Notation** (postfix notation). Evaluate the expression and return an integer.
Rules:
- The valid operators are `+`, `-`, `*`, and `/`.
- Each operand may be an integer or another expression.
- Division between two integers always **truncates toward zero**.
- There will be no division by zero.
- The answer and all intermediate calculations fit in a 32-bit integer.
- 1 <= tokens.length <= 10^4
- tokens[i] is either an operator ("+", "-", "*", "/") or an integer in the range [-200, 200].
- The input always represents a valid RPN expression.