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

我的C++实现代码如下：

string addBinary(string a, string b) {
    string sum;
    int carry = 0;
    for (int i = a.size() - 1, j = b.size() - 1; i >= 0 || j >= 0; --i, --j) {
        string bitAdd("0");
        int src1 = (i >= 0 ? a[i] - '0' : 0);
        int src2 = (j >= 0 ? b[j] - '0' : 0);
        bitAdd[0] +=  (src1 + src2 + carry) & 1;
        carry = ((src1 + src2 + carry) & 2) ? 1 : 0;
        sum = bitAdd + sum;
    }

    if (carry) {
        string s("1");
        sum = s + sum;
    }

    return sum;
}

LeetCode[String]: Add Binary