code: public Set<Map.Entry<String, V>> prefixSearch(String key)
      {
          Set<Map.Entry<String, V>> entrySet = new TreeSet<Map.Entry<String, V>>();
          StringBuilder sb = new StringBuilder(key.substring(0, key.length() - 1));
          BaseNode branch = this;
          char[] chars = key.toCharArray();
          for (char aChar : chars)
          {
              if (branch == null) return entrySet;
              branch = branch.getChild(aChar);
          }
  
          if (branch == null) return entrySet;
          branch.walk(sb, entrySet);
          return entrySet;
      }
docs: 前缀查询
  
  @param key 查询串
  @return 键值对