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

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

Solution: 不要忘记剩下的进位

    public String addBinary(String a, String b) {
        if (a == null || b == null) {
            return "";
        }
        StringBuilder sb = new StringBuilder();
        int i = a.length() - 1, j = b.length() - 1, extra = 0;
        while (i >= 0 && j >= 0) {
            int sum = a.charAt(i) - '0' + b.charAt(j) - '0' + extra;
            extra = sum / 2;
            sum %= 2;
            sb.append(sum);
            i--;
            j--;
        }
        while (i >= 0) {
            int sum = a.charAt(i) - '0' + extra;
            extra = sum / 2;
            sum %= 2;
            sb.append(sum);
            i--;
        }
        while (j >= 0) {
            int sum = b.charAt(j) - '0' + extra;
            extra = sum / 2;
            sum %= 2;
            sb.append(sum);
            j--;
        }
        //记得
        if (extra != 0) {
            sb.append(extra);
        }
        return sb.reverse().toString();
    }