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));

    }