43. Multiply Strings

public class Solution {
    public String multiply(String num1, String num2) {
        int[] digits=new int[num1.length()+num2.length()];
        for(int i=0;i<num1.length();i++)
            for(int j=0;j<num2.length();j++)
                digits[digits.length-i-j-1]+=(num1.charAt(num1.length()-i-1)-'0')*(num2.charAt(num2.length()-j-1)-'0');
        StringBuilder sb=new StringBuilder();
        for(int i=digits.length-1;i>=0;i--)
        {
            if(i>0)
                digits[i-1]+=digits[i]/10;
            sb.insert(0,(char)('0'+digits[i]%10));
        }
        int idx=0;
        while(idx<sb.length()-1&&sb.charAt(idx)=='0')
            idx++;
        return sb.substring(idx).toString();
    }
}