使用字符串实现整数相乘——绝对精确和支持无限大

StringBuilder multiplyTwoIntegerNum(StringBuilder num1, StringBuilder num2){
    int length1 = num1.length();
    int length2 = num2.length();
    int[] multiplyArray = new int[length1 + length2];
    for (int i = length1 - 1; i > -1; i--) {
        for (int j = length2 - 1; j > -1; j--) {
            multiplyArray[i + j] += (num1.charAt(i) - '0') * (num2.charAt(j) - '0');
        }
    }

    StringBuilder result = new StringBuilder();
    for (int i = multiplyArray.length - 2; i > 0; i--) {
        int d = multiplyArray[i] % 10;
        int c = multiplyArray[i] / 10;
        multiplyArray[i - 1] += c;
        result.insert(0, d);
    }
    result.insert(0, multiplyArray[0]);
    return result;
}