text: function removeDuplicateCharacters(str) {
      const result = [];
      let store = new Map();
      
      for (const letter of str) {
          if (store.has(letter)) {
              store.set(letter, 1 + store.get(letter));
          } else {
              store.set(letter, 1);
          }
      }
      
      for (const letter of str) {
          if (1 === store.get(letter)) {
              result.push(letter);
          }
      }
      
      return result.join('');
  }
  