ul { /* 取消ul預設的內縮及樣式 */ 
        margin: 0;
        padding: 0;
        list-style: none;
    }

    ul.drop-down-menu {
        display: inline-block;
    }

    ul.drop-down-menu li {
        position: relative;
        white-space: nowrap;
    }

    ul.drop-down-menu > li:last-child {
        border-right: none;
    }

    ul.drop-down-menu > li {
        float: left; /* 只有第一層是靠左對齊*/
    }  

		ul.drop-down-menu a {
        display: block;
        padding: 0 30px;
        text-decoration: none;
        line-height: 40px;
        
    }
    ul.drop-down-menu a:hover { /* 滑鼠滑入按鈕變色*/

        color: #fff;
    }
    ul.drop-down-menu li:hover > a { /* 滑鼠移入次選單上層按鈕保持變色*/

        color: #fff;
		}

		ul.drop-down-menu ul {
        position: absolute;
        z-index: 999;
        left: -1px;
        top: 100%;
       min-width: 180px;
    }

    ul.drop-down-menu ul li {
        border-bottom: #ccc 1px solid;
				background-color: rgba(160, 160, 160, 0.5);
    }

    ul.drop-down-menu ul li:last-child {
        border-bottom: none;
    }

    ul.drop-down-menu ul ul { /*第三層以後的選單出現位置與第二層不同*/
        z-index: 999;
        top: 10px;
        left: 90%;
    }
		  ul.drop-down-menu ul { /*隱藏次選單*/
        display: none;
    } 

    ul.drop-down-menu li:hover > ul { /* 滑鼠滑入展開次選單*/
        display: block;
    }
		  ul.drop-down-menu ul { /*隱藏次選單*/  
        left: 99999px;
        opacity: 0;
        -webkit-transition: opacity 0.3s;
        transition: opacity 0.3s;        
    } 

    ul.drop-down-menu li:hover > ul { /* 滑鼠滑入展開次選單*/
        opacity: 1;
        -webkit-transition: opacity 0.3s;
        transition: opacity 0.3s; 
        left: -1px;
        border-right: 5px;
    }
    
    ul.drop-down-menu li:hover > ul ul { /* 滑鼠滑入之後、次選單之後的選單依舊隱藏*/
        left: 99999px;
    } 
    
    ul.drop-down-menu ul li:hover > ul { /* 第二層之後的選單展開位置*/
        left: 90%;
    }