Multiply Strings

大数相乘， 按位算结果， 注意全0，注意进位

public class Solution {
    public String multiply(String num1, String num2) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        int length1 = num1.length();
        int length2 = num2.length();
        int[] result = new int[length1 + length2 + 1];
        int resultlength = length1 + length2 + 1;
        for(int i = 0; i < length1; i++)
        {
            int carry = 0;
            int offset = resultlength - 1 - i;
            for(int j = 0; j < length2; j++)
            {
                int tmp1 = num1.charAt(length1 - 1 - i) - '0';
                int tmp2 = num2.charAt(length2 - 1 - j) - '0';
                int tmpresult = tmp1 * tmp2 + carry + result[offset - j];
                result[offset - j] = tmpresult % 10;
                carry = tmpresult / 10;
            }
            if(carry > 0)
            {
                int tmp = result[offset - length2] + carry;
                result[offset - length2] = tmp % 10;
                result[offset - length2 - 1] = tmp / 10;
            }
        }
        
        StringBuilder myresult = new StringBuilder();
        int i = 0;
        while(i < resultlength && result[i] == 0)
            i++;
        if(i >= resultlength)
            return "0";
        while(i < resultlength)
        {
            myresult.append(String.valueOf(result[i]));
            i++;
        }
        return myresult.toString();
    }
}