add-binary 字符串操作，二进制字符串相加

Given two binary strings, return their sum (also a binary string).

For example,
a ="11"
b ="1"
Return"100".

思路很简单，先把短的字符串补齐，然后逐位相加。
string addBinary(string a, string b)
{
    int length = max(a.size(), b.size());
    string res(length + , ' ');
    char flag = '';
    while (length > a.size())
    {
        a = "" + a;
    }
    while (length > b.size())
    {
        b = "" + b;
    }
    for (int i = length - ; i >= ; --i)
    {
        int ch = a[i] + b[i] + flag -  * '';
        if (ch == )
        {
            res[i + ] = ''; flag = '';
        }
        else if (ch == )
        {
            res[i + ] = ''; flag = '';
        }
        else
        {
            res[i + ] = ch + '';
            flag = '';
        }
    }
    if (flag == '') res[] = '';
    else res = res.substr();
    return res;
}