大数乘法

static String bigNumMul(String num1,String num2) {
        char num1Chars[] = num1.toCharArray();
        char num2Chars[] = num2.toCharArray();
        int num1Ary[] = new int[num1Chars.length];
        for (int i = 0; i < num1Ary.length; i++) {
            num1Ary[i] = num1Chars[i] - '0';
        }
        
        int num2Ary[] = new int[num2Chars.length];
        for (int i = 0; i < num2Ary.length; i++) {
            num2Ary[i] = num2Chars[i] - '0';
        }
        
        int resAry[] = new int[num1Chars.length + num2Chars.length - 1];
        for (int i = 0; i < num1Ary.length; i++) {
            for (int j = 0; j < num2Ary.length; j++) {
                resAry[i + j] += num1Ary[i] * num2Ary[j];
            }
        }
        
        for (int i = resAry.length - 1; i > 0; i--) {
            resAry[i - 1] += resAry[i] / 10;
            resAry[i] = resAry[i] % 10;
        }
        int shouwei = resAry[0] / 10;
        resAry[0] = resAry[0] % 10;
        
        String res = shouwei == 0 ? "" : "1";
        for (int i = 0; i < resAry.length; i++) {
            res += resAry[i];
        }
        return res;
    }