ps:题目来自力扣

给你两个二进制字符串 a 和 b ，以二进制字符串的形式返回它们的和。

class Solution {
    public String addBinary(String a, String b) {
        StringBuilder result = new StringBuilder();
        int i = a.length() - 1;
        int j = b.length() - 1;
        int carry = 0;

        while (i >= 0 || j >= 0) {
            int sum = carry;
            if (i >= 0) {
                sum += a.charAt(i) - '0';
                i--;
            }
            if (j >= 0) {
                sum += b.charAt(j) - '0';
                j--;
            }
            result.append(sum % 2);
            carry = sum / 2;
        }

        if (carry != 0) {
            result.append(carry);
        }

        return result.reverse().toString();
    }
}