67.二进制求和

方法：模拟计算

class Solution {
    public String addBinary(String a, String b) {
        StringBuilder ans = new StringBuilder();
        int carry = 0;
        for(int i=a.length()-1,j=b.length()-1;i>=0||j>=0;i--,j--){
            int sum = carry;
            sum += i >=0 ? a.charAt(i) - '0': 0;
            sum += j >= 0 ? b.charAt(j) - '0': 0;
            ans.append(sum % 2);
            carry = sum / 2;
        }
        ans.append(carry == 1 ? carry : "");
        return ans.reverse().toString();
    }
}