solution.cpp

class Solution 
{
public:
    string addBinary(string a, string b) 
    {
        string c;
        int flag=0;
        int lena = a.size();
        int lenb = b.size();
        int len = abs(lena-lenb);
        string append(len,'0');
        if(lena>lenb)
        {
            b = append + b;
            c.resize(lena,'0');
        }
        else
        {
            a = append + a;
            c.resize(lenb,'0');
        }
        for(int j=c.size()-1;j>=0;j--)
        {
            int current = (a[j]-'0') ^(b[j]-'0') ^flag;
            if((a[j]-'0') +(b[j]-'0') +flag >1)
                flag = 1;
            else 
                flag = 0;
            c[j] = current+'0';
        }
        if(flag == 1)
            c = '1'+ c;
        return c;
    }
};