function getFullByRegExp(reg) {
    let res = "";
    while (ind < expr.length && reg.test(expr[ind])) {
        res += expr[ind];
        ind++;
    }
    return res;
}

function skipWhiteSpaces() {
    while (ind < expr.length && /\s/.test(expr[ind])) {
        ind++;
    }
}

function getOperation() {
    skipWhiteSpaces();
    switch (expr[ind]) {
        case '-':
        case '+':
        case '/':
        case '*':
            return expr[ind];
        default :
        // ошибка
    }
}

function getOperand() {
    let curStack = [];
    skipWhiteSpaces();
    switch (expr[ind]) {
        case '(':
            ind++;
            let symb = getOperation();
            curStack.push(symb);
            ind++;
            let op1 = getOperand();
            ind++;
            let op2 = getOperand();
            for (let i = 0; i < op2.length; i++){
                curStack.push(op2[i]);
            }
            for (let i = 0; i < op1.length; i++){
                curStack.push(op1[i]);
            }
            break;
        case ')':
            ind++;
            break;
        default :
            let st = [];
            if (/[A-Za-z]/.test(expr[ind])) {
                st.push(getFullByRegExp(/[A-Za-z]/));
                return st;
            } else if (/\d/.test(expr[ind])) {
                st.push(parseInt(getFullByRegExp(/\d/)));
                return st;
            } else {
                // error
            }
    }
    return curStack;
}

const parsePrefix = function (str) {
    ind = 0;
    expr = str;
    skipWhiteSpaces();
    switch (expr[ind]) {
        case '(':
            ind++;
            let symb = getOperation();
            stack.push(symb);
            ind++;
            let op1 = getOperand();
            ind++;
            let op2 = getOperand();
            for (let i = 0; i < op2.length; i++){
                stack.push(op2[i]);
            }
            for (let i = 0; i < op1.length; i++){
                stack.push(op1[i]);
            }
            break;
    }
    let res = "";
    while (stack.length > 0) {
        res += stack.pop();
        res += " ";
    }
    // return parse(res);
    return res;
};