67 Add Binary

Given two binary strings, return their sum (also a binary string).
For example,
a = "11", b = "1"
Return "100".

Solution:

class Solution {
public:
    string addBinary(string a, string b) {
        int na = a.size(), nb = b.size();
        int ia, ib, ic = 0;
        string res;
        for ( int i = 0; i <= max(na, nb)-1; i++ ) {
            if ( na-1-i <= -1 ) ia = 0;
            else {
                string sa = a.substr(na-1-i, 1);
                ia = atoi(sa.c_str());
            }
            if ( nb-1-i <= -1 ) ib = 0;
            else {
                string sb = b.substr(nb-1-i, 1);
                ib = atoi(sb.c_str());
            }
            cout << ia << ib << ic << endl;
            int sum = ia + ib + ic;
            if ( sum == 3 ) { res += '1'; ic = 1;}
            if ( sum == 2 ) { res += '0'; ic = 1;}
            if ( sum == 1 ) { res += '1'; ic = 0;}
            if ( sum == 0 ) { res += '0'; ic = 0;}
        }
        if ( ic == 1 ) res += '1';
        int i = 0, j = res.size()-1;
        while ( i <= j ) swap(res[i++], res[j--]);
        return res;
    }
};

Notes: cast a character (or string) to integer.

// num = int(s[i])
string subStr = s.substr(i, 1);
int num = atoi(subStr.c_str());