code: protected static void addFeature(String feature, FeatureMap featureMap, List<Integer> featureList)
      {
          int featureId = featureMap.idOf(feature);
          if (featureId != -1)
              featureList.add(featureId);
      }
docs: 向特征向量插入特征
  
  @param feature     特征
  @param featureMap  特征映射
  @param featureList 特征向量