67.二进制求和 (Easy)*

class Solution {
public:
    string addBinary(string a, string b) {
        string s;
        s.reserve(a.length() + b.length());
        int i = a.length() - 1, j = b.length() - 1;
        int res = 0;
        while(i >= 0 || j >= 0 || res == 1) {
            res += (i >= 0) ? a[i--] - '0' : 0;
            res += (j >= 0) ? b[j--] - '0' : 0;
            s.push_back((res & 1) + '0');
            res >>= 1;
        }
        reverse(s.begin(), s.end());
        return s;
    }
};