Add binary

public String addBinary(String a, String b) {
          StringBuilder res = new StringBuilder();
          int i = a.length() - 1, j = b.length() - 1, carry = 0;
          while (i >= 0 || j >= 0) {
              int sum = carry;
              // char -> ASCII value: '0' -> 48, '1' -> 49,
              // e.g., subtract '0' from '1' and we have 1
              if (i >= 0) sum += a.charAt(i--) - '0';
              if (j >= 0) sum += b.charAt(j--) - '0';
              carry = sum / 2;
              res.insert(0, String.valueOf(sum % 2));
          }

          if (carry == 1) res.insert(0, '1');
          return res.toString();
      }

Roman to Int

public int romanToInt(String s) {
          HashMap<Character, Integer> map = new HashMap<Character, Integer>() {{
              put('I', 1);
              put('V', 5);
              put('X', 10);
              put('L', 50);
              put('C', 100);
              put('D', 500);
              put('M', 1000);
          }};

          int res = 0;
          for (int i = 0; i < s.length(); i++) {
              // add guard condition to avoid the StringIndexOutOfBoundsException when grap the next character
              if (i < s.length() - 1 && map.get(s.charAt(i)) < map.get(s.charAt(i + 1)))
                  res -= map.get(s.charAt(i));
              else res += map.get(s.charAt(i));
          }

          return res;
      }