Add Binary

Description：

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

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

Code：

    string addBinary(string a, string b) {
    //4ms
    const char * shortStr = (a.size()<=b.size())?a.c_str():b.c_str();
    const char * longStr = (a.size()>b.size())?a.c_str():b.c_str();
    int lenShort = strlen(shortStr);
    int lenLong = strlen(longStr);
    
    string s="";
    int temp;
    int flag = 0;
    for (int i = 0; i < lenShort; ++i)
    {
        temp = shortStr[lenShort-1-i]-'0'+longStr[lenLong-1-i]-'0'+ flag;
        flag = temp/2;
        s.insert(0,1,'0'+temp%2);
    }
   
    for (int i = lenLong-1-lenShort; i >=0; --i)
    {
        temp = longStr[i]-'0'+flag;
        flag = temp/2;
        s.insert(0,1,'0'+temp%2);
    }
    
    if (flag == 1)
        s.insert(0,1,'1');
    return s;
    }