code: public static String mergeSeq(final String first, final String second, final String delimiter) {
      if (isNotEmpty(second) && isNotEmpty(first)) {
        List<String> firstSeq = Arrays.asList(split(first, delimiter));
        List<String> secondSeq = Arrays.asList(split(second, delimiter));
        Collection<String> rs = CollectUtils.union(firstSeq, secondSeq);
        StringBuilder buf = new StringBuilder();
        for (final String ele : rs)
          buf.append(delimiter).append(ele);
        if (buf.length() > 0) buf.append(delimiter);
        return buf.toString();
      } else {
        return ((first == null) ? "" : first) + ((second == null) ? "" : second);
      }
    }
docs: 将两个用delimiter串起来的字符串，合并成新的串，重复的"单词"只出现一次.
  如果第一个字符串以delimiter开头，第二个字符串以delimiter结尾，<br>
  合并后的字符串仍以delimiter开头和结尾.<br>
  <p>
  <blockquote>
  
  <pre>
  mergeSeq(&quot;,1,2,&quot;, &quot;&quot;) = &quot;,1,2,&quot;;
  mergeSeq(&quot;,1,2,&quot;, null) = &quot;,1,2,&quot;;
  mergeSeq(&quot;1,2&quot;, &quot;3&quot;) = &quot;1,2,3&quot;;
  mergeSeq(&quot;1,2&quot;, &quot;3,&quot;) = &quot;1,2,3,&quot;;
  mergeSeq(&quot;,1,2&quot;, &quot;3,&quot;) = &quot;,1,2,3,&quot;;
  mergeSeq(&quot;,1,2,&quot;, &quot;,3,&quot;) = &quot;,1,2,3,&quot;;
  </pre>
  
  </blockquote>
  <p>
  
  @param first
  a {@link java.lang.String} object.
  @param second
  a {@link java.lang.String} object.
  @param delimiter
  a {@link java.lang.String} object.
  @return a {@link java.lang.String} object.