1106. Parsing A Boolean Expression

class Solution {
    public boolean parseBoolExpr(String expression) {
        return helper(expression, 0, expression.length() - 1);
    }

    private boolean helper(String e, int lo, int hi) {
        if (lo == hi) {
            return e.charAt(lo) == 'f' ? false : true;
        }

        char op = e.charAt(lo);
        boolean res = (op == '|') ? false : true;

        int count = 0;
        for (int i = lo + 1, prev = lo + 2; i <= hi; i++) {
            char c = e.charAt(i);

            if (c == '(') {
                count++;
            } else if (c == ')') {
                count--;
            }

            if ((c == ',' && count == 1) ||
                (c == ')' && count == 0)) {
                boolean next = helper(e, prev, i - 1);

                prev = i + 1;
                if (op == '|') {
                    res |= next;
                } else if (op == '&') {
                    res &= next;
                } else {
                    res = !next;
                }
            }
        }

        return res;
    }
}