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 + 1, ' ');
    char flag = '0';
    while (length > a.size())
    {
        a = "0" + a;
    }
    while (length > b.size())
    {
        b = "0" + b;
    }
    for (int i = length - 1; i >= 0; --i)
    {
        int ch = a[i] + b[i] + flag - 3 * '0';
        if (ch == 3)
        {
            res[i + 1] = '1'; flag = '1';
        }
        else if (ch == 2)
        {
            res[i + 1] = '0'; flag = '1';
        }
        else
        {
            res[i + 1] = ch + '0';
            flag = '0';
        }    
    }
    if (flag == '1') res[0] = '1';
    else res = res.substr(1);
    return res;
}