43. 字符串相乘

class Solution {     public String multiply(String num1, String num2) {         int[] res = new int[num1.length() + num2.length()];         int temp = 0;         for (int i = num1.length() - 1; i >= 0; i--)             for (int j = num2.length() - 1; j >= 0; j--) {                 temp = (num1.charAt(i) - '0') * (num2.charAt(j) - '0') + res[i + j + 1];                 res[i + j] += temp / 10;                 res[i + j + 1] = temp % 10;             }         StringBuilder sb=new StringBuilder();             int index=0;          while (index<res.length&&res[index]==0){              index++;          }          for (int i=index;i<res.length;i++)              sb.append(res[i]);          return sb.length()==0?"0":sb.toString();     } }