<style>
        * {
            margin: 0;
            padding: 0;
        }

        li {
            list-style-type: none;
        }

        .classify>ul>li {
            width: 200px;
            text-indent: 20px;
            background: blue;
        }

        .classify>ul>li>ul>li {

            background: #fff;
        }

        .classify>ul>li ul {
            display: none;
        }

        .active {
            /*!important优先级最高，必不可少，否则执行上一行代码display:none隐藏ul*/
            display: block!important;
        }
    </style>
    <script>
        window.onload = function () {
            var li = document.querySelectorAll(".classify>ul>li"),
                ul = document.querySelectorAll(".classify>ul>li>ul"),
                index = null;
            for (var i = 0, len = li.length; i < len; i++) {
                li[i].index = i;
                li[i].onclick = function () {
                    for (var j = 0, len2 = ul.length; j < len2; j++) {
                        //除了当前显示的ul添加active,其他ul都删除active  
                        if (this.index != j) ul[j].className = "";
                    }
                    //第一次点击时添加active,显示ul
                    if (ul[this.index].className == "")
                        ul[this.index].className = "active";
                    else
                    //第二点击时删除active,隐藏ul
                        ul[this.index].className = "";
                }
            }
        }

    </script>

</head>

<body>
    <div class="classify">
        <ul>
            <li>
                <p>1</p>
                <ul>
                    <li>11</li>
                    <li>12</li>
                    <li>13</li>
                </ul>
            </li>
            <li>
                <p>2</p>
                <ul>
                    <li>22</li>
                    <li>2</li>
                    <li>2</li>
                </ul>
            </li>
            <li>
                <p>3</p>
                <ul>
                    <li>3</li>
                    <li>3</li>
                    <li>3</li>
                </ul>
            </li>
            <li>
                <p>4</p>
                <ul>
                    <li>4</li>
                    <li>4</li>
                    <li>4</li>
                </ul>
            </li>
        </ul>
    </div>
</body>

</html>