67. Add Binary

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

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

题目含义：将两个字符串进行二进制加减法。

思路：从最后一位开始依次计算，注意进位

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