Bootstrap的下拉菜单float问题

<div class="dropdown">
  <button class="btn btn-default dropdown-toggle" type="button" id="dropdownMenu1" data-toggle="dropdown">
  下拉菜单
  <span class="caret"></span>
  </button>
  <ul class="dropdown-menu pull-right" role="menu" aria-labelledby="dropdownMenu1">
   …
  </ul>
</div>

.dropdown-menu.pull-right {
  right: 0;
  left: auto;
}
.dropdown-menu-right {
  right: 0;
  left: auto;
}