Multiply Strings 大数相乘 java

先贴上代码

 public String multiply(String num1, String num2) {

         String str = "";

         StringBuffer sb = new StringBuffer(num1);

         num1 = sb.reverse().toString();

         sb = new StringBuffer(num2);

         num2 = sb.reverse().toString();

         int[] res = new int[num1.length() + num2.length()];

         for (int i = 0; i < num1.length(); i++) {

             for (int j = 0; j < num2.length(); j++) {

                 res[i + j] += (num1.charAt(i) - '0') * (num2.charAt(j) - '0');

             }

         }

         int k = 0, contribute = 0;

         while (k < res.length || contribute > 0) {

             int tmp = k<res.length?res[k]:0;

             tmp = contribute+tmp;

             contribute = tmp/10;

             str += tmp%10;

             k++;

         }

         sb = new StringBuffer(str);

         str = sb.reverse().toString();

         int i = 0;

         while(i<str.length()&&str.charAt(i)=='0')

             i++;

         if(i<str.length()){

             str = str.substring(i);

         }else{

             str = "0";

         }

         return str;

     }

思路如下：

模拟竖式相乘

1、转换并反转，字序反转；
2、逐位相乘，结果存放在res[i+j]中；
3、处理进位；
4、转换并反转，将计算结果转换为字符串并反转。
5、消除多余的0；

两数相乘，结果的长度一定不大于乘数和被乘数的长度之和。

上述也可以用直接用java的BigInteger类实现

public static void main(String[] args) {

        String a = "98989898989898956898", b = "989892551548781251323265615150";

        BigInteger aa = new BigInteger(a);

        BigInteger bb = new BigInteger(b);

        System.out.println(aa.multiply(bb));

    }