Evaluate Reverse Polish Notation
RPN (postfix) puts operators after their operands, so no parentheses are ever needed, which is exactly why calculators and compilers use it. Write evalRPN(tokens) that evaluates an array of tokens (numbers as strings, plus "+", "-", "*", "/").
Use a stack: push numbers; on an operator, pop the top two, apply, push the result. Division should truncate toward zero (integer division).
evalRPN(["2", "1", "+", "3", "*"]); // 9 ((2 + 1) * 3)
evalRPN(["4", "13", "5", "/", "+"]); // 6 (4 + (13 / 5 → 2))