如何使用css和flexbox在所选侧导航项的末尾显示箭头？

<ul class="nav-menu">
  <li>导航项1</li>
  <li>导航项2</li>
  <li>导航项3</li>
</ul>

.nav-menu {
  display: flex;
  flex-direction: column;
  list-style-type: none;
  padding: 0;
}

.nav-menu li {
  display: flex;
  align-items: center;
  padding: 10px;
}

.nav-menu li::after {
  content: "";
  width: 0;
  height: 0;
  border-style: solid;
  border-width: 5px 5px 0 5px;
  border-color: #000 transparent transparent transparent;
  margin-left: auto;
}

<ul class="nav-menu">
  <li>导航项1</li>
  <li class="selected">导航项2</li>
  <li>导航项3</li>
</ul>

.nav-menu li.selected::after {
  border-color: #000 transparent transparent transparent;
}