67、Add Binary

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

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

 Math String

string addBinary(string a, string b) {        //此题比较简单        int i1 = a.size() - 1, i2 = b.size() - 1;        int tmp = 0, sum;        string str;        while (i1 >= 0 && i2 >= 0)        {            sum = a[i1] - '0'  + b[i2] - '0' + tmp;            str.push_back(sum % 2 + '0');            tmp = sum / 2;            --i1;            --i2;        }        while (i1 >= 0)        {            sum = a[i1] - '0' + tmp;            str.push_back(sum % 2 + '0');            tmp = sum / 2;            --i1;        }         while (i2 >= 0)        {            sum = b[i2] - '0' + tmp;            str.push_back(sum % 2 + '0');            tmp = sum / 2;            --i2;        }        if (tmp)            str.push_back(tmp + '0');        reverse(str.begin(), str.end());        if(str.empty())            str.push_back('0');        return str;    }