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export default function evalRPN(tokens: string[]): number {    const stack: (string | number)[] = [];
    for (const char of tokens) {        if (isNumber(char)) {            stack.push(Number(char));        } else {            const num2 = stack.pop();            const num1 = stack.pop();            if (typeof num2 !== "number") {                throw Error("number expected");            }            if (typeof num1 !== "number") {                throw Error("number expected");            }            if (char === "+") {                stack.push(num1 + num2);            } else if (char === "-") {                stack.push(num1 - num2);            } else if (char === "*") {                stack.push(num1 * num2);            } else if (char === "/") {                const sign = Math.sign(num2) * Math.sign(num1);                stack.push(sign * Math.floor(Math.abs(num1) / Math.abs(num2)));            } else throw Error("unknown operator:" + char);        }    }    return Number(stack[0]);}export function isNumber(token: string) {    return /\d+/g.test(token);}