// Evaluate Reverse Polish Notation — MEDIUM
// Category: stack
You are given an array of strings `tokens` that represents an arithmetic expression in **Reverse Polish Notation** (RPN).
Evaluate the expression. Return an integer that represents the value.
**Valid operators:** `+`, `-`, `*`, `/`. Each operand may be an integer or another expression. Division truncates toward zero.
Example: tokens = ["2","1","+","3","*"]
Output: 9