.nav-dropdown{
}
/* 菜单 */
.nav-dropdown ul{
   display: none;
   padding: 0 20px !important;
}
/* 第一级菜单 */
.nav-dropdown > ul{
  left:0;
}
.nav-dropdown:hover > ul{
  display: block;
}
/* 所有菜单项 */
.nav-dropdown li{
  position: relative;
  color: #000;
  width: 200px;
}
.nav-dropdown ul li ul{
  left: 100%;
  top: 0;
  overflow: hidden; 
  overflow-y: auto;
  max-height: 360px;
}
.nav-dropdown ul li:hover > ul{
  display: block;
}