67. 二进制求和

输入: a = "11", b = "1"
输出: "100"

输入: a = "1010", b = "1011"
输出: "10101"

class Solution {
    public String addBinary(String a, String b) {
        StringBuilder result = new StringBuilder();
        int pointerA = a.length() - 1;
        int pointerB = b.length() - 1;
        //进位标志
        int carry = 0;
        while (pointerA >= 0 || pointerB >= 0) {
            int sum = carry;
            if (pointerA >= 0) {
                sum += (a.charAt(pointerA) - '0');
                pointerA--;
            }
            if (pointerB >= 0) {
                sum += (b.charAt(pointerB) - '0');
                pointerB--;
            }
            result.append(sum % 2);
            carry = sum / 2;
        }
        if (carry != 0) {
            result.append('1');
        }
        return result.reverse().toString();
    }
}