code: function findSingleVacancies(str, size) {
      var vacs = [];
      for (var i = 0; i < CANDIDATE_CHARS.length; i++) {
          if (str.indexOf(CANDIDATE_CHARS[i]) < 0) {
              vacs.push(CANDIDATE_CHARS[i]);
              if (size && vacs.length >= size) break;
          }
      }
      return vacs;
  }
docs: Find single-character vacancies of a string
  @param {string} str
  @param {number} [size]
  @returns {string[]}