两个大数string相加

string account(string num1, string num2)
{
    if (num1.size()<num2.size())
    {
        string temp = num1;
        num1 = num2;
        num2 = temp;
    }
    int length1 = num1.size(), length2 = num2.size(), flag = 0, a, b, sum;
    while (length1>0)
    {
        a = num1[length1 - 1] - '0';
        if (length2>0)
            b = num2[length2 - 1] - '0';
        else
            b = 0;
        sum = a + b + flag;
        if (sum >= 10)
        {
            num1[length1 - 1] = '0' + sum % 10;
            flag = 1;
        }
        else
        {
            num1[length1 - 1] = '0' + sum;
            flag = 0;
        }
        length1--;
        length2--;
    }
    if (1 == flag)
        num1 = "1" + num1;
    return num1;
}