function separateDigitsAndLetters(inputString) {
    let digitsString = '';
    let lettersString = '';

    for (let char of inputString) {
        if (char >= '0' && char <= '9') {
            digitsString += char;
        } else if ((char >= 'a' && char <= 'z') || (char >= 'A' && char <= 'Z')) {
            lettersString += char;
        }
    }

    return { digitsString, lettersString };
}

// 测试代码
const inputString = "a1b2c3d4e5f6g7h8i9";
const result = separateDigitsAndLetters(inputString);

console.log("数字字符串:", result.digitsString);
console.log("字母字符串:", result.lettersString);