leetcode 67 add binaies

class {
  public String addBinaires(String a, String b){
    int i = a.length()-1;
    int j = b.length()-1;
    StringBuilder result = new StringBuilder();
    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';
      }
      result.append(sum%2);
      carry = sum / 2;
    }
    if(carry != 0){
      result.append(carry);
    }
    return result.reverse().toString();
  }
}