Центрировать корень многостороннего дерева в HTML/CSS

* {
  margin: 0;
  padding: 0;
}

.tree ul {
  padding-top: 20px;
  position: relative;
}

/* Using display: inline-block to limit its width */

.tree > ul {
  display: inline-block;
  transform: rotate(90deg);
}

/* Clearing floats (so <ul> has a defined height) */

.tree ul:after {
  content: "";
  display: table;
  clear: both;
}

.tree li {
  float: left;
  text-align: center;
  list-style-type: none;
  position: relative;
  padding: 20px 5px 0 5px;
}

.tree li::before,
.tree li::after {
  content: '';
  position: absolute;
  top: 0;
  right: 50%;
  border-top: 1px solid #ccc;
  width: 50%;
  height: 20px;
}

.tree li::after {
  right: auto;
  left: 50%;
  border-left: 1px solid #ccc;
}

.tree li:only-child::after,
.tree li:only-child::before {
  display: none;
}

.tree li:only-child {
  padding-top: 0;
}

.tree li:first-child::before,
.tree li:last-child::after {
  border: 0 none;
}

.tree li:last-child::before {
  border-right: 1px solid #ccc;
  border-radius: 0 5px 0 0;
}

.tree li:first-child::after {
  border-radius: 5px 0 0 0;
}

.tree ul ul::before {
  content: '';
  position: absolute;
  top: 0;
  left: 50%;
  border-left: 1px solid #ccc;
  width: 0;
  height: 20px;
}

.tree li a {
  border: 1px solid #ccc;
  border-radius: 25px 25px 25px 25px;
  padding: 5px 10px;
  text-decoration: none;
  color: #666;
  font-family: arial, verdana, tahoma;
  font-size: 11px;
  display: inline-block;
}

.tree .special {
  min-width: 94.594px;
}

.tree .special > ul > li {
  float: none;
}

<div class = "tree">
  <ul>
    <li>
      <a href = "#">1</a>
      <ul>
        <li>
          <a href = "#">2</a>
          <ul>
            <li>
              <a href = "#">2.1</a>
            </li>
            <li>
              <a href = "#">2.2</a>
            </li>
          </ul>
        </li>
        <li>
          <a href = "#">3</a>
          <ul>
            <li>
              <a href = "#">3.1</a>
            </li>
            <li>
              <a href = "#">3.2</a>
            </li>
          </ul>
        </li>
        <li class = "special">
          <a href = "#">4</a>
          <ul>
            <li>
              <a href = "#">4.1</a>
            </li>
          </ul>
        </li>
      </ul>
    </li>
  </ul>
</div>