// dom几点如下
<ul id="pid_0">
    <li>
        <input type="text" name="classify" class="form-control one_line" value="一级目录" pid="0" level="1">
        <span class="input_sign add first_add" pid="0" sign="add" level="0" ref="child" style="display: none;">[ + ]</span>

        <ul id="pid_1">
            <li>
                <input class="form-control one_line" pid="0" level="2" name="classify">
                <span class="input_sign add" pid="1" level="2" sign="add" ref="child"
                      style="display: none;">[ + ]</span>
                <span class="input_sign reduce" pid="1" level="2" sign="reduce" ref="sibling">[ - ]</span>
                <ul id="pid_2">
                    <li>
                        <input class="form-control one_line" pid="1" level="3" name="classify">
                        <span class="input_sign add" pid="2" level="3" sign="add" ref="child">[ + ]</span>
                        <span class="input_sign reduce" pid="2" level="3" sign="reduce" ref="sibling">[ - ]</span>
                    </li>
                    <li>
                        <span class="input_sign add" pid="1" level="2" sign="add" ref="sibling"> + </span>
                    </li>
                </ul>
            </li>
            <li>
                <input class="form-control one_line" pid="0" name="classify" level="2">
                <span class="input_sign add" sign="add" ref="child" pid="1" level="2">[ + ]</span>
                <span class="input_sign reduce" pid="0" level="1" sign="reduce" ref="child">[ - ]</span>
            </li>
            <li>
                <span class="input_sign add" pid="0" level="1" sign="add" ref="sibling"> + </span>
            </li>
        </ul>
    </li>

    <li id="lastChild_0"><span class="input_sign" pid="0" sign="add" ref="sibling" level="0"> + </span></li>

</ul>