• 大数乘法

public class BigInteger {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String str1 = scanner.next();
        String str2 = scanner.next();
        scanner.close();

        int ret[] = new int[str1.length() + str2.length()];
        //72106547548473106236 982161082972751393
        for(int i = str1.length() - 1; i >= 0; --i){
            for(int j = str2.length() - 1; j >= 0; --j){
                int num1 = str1.charAt(i) - '0';
                int num2 = str2.charAt(j) - '0';

                ret[i+j] += (ret[i+j+1] + num1 * num2)/10;//十位：就是上一次的个位 + 进位
                ret[i+j+1] = (ret[i+j+1] + num1 * num2)%10;//个位：就是个位
            }
        }

        StringBuilder builder = new StringBuilder();
        for(int i = 0; i < ret.length; ++i){
            if(i == 0 && ret[i] == 0)
                continue;
            builder.append(ret[i]);
        }
        System.out.println(builder.toString());
    }
}