By searching on the forum, I have found something similar what I needed. I would like my menu to display subnodes if there any, if not, only parent nodes. but in the following code, I should first click parent page, then it displays subpage. How could I achieve what I need?

-Page1

   -Page 1.1

   -Page 1.2

Page2

<?xml version="1.0" encoding="utf-8" ?>
<xsl:stylesheet
        version="1.0"
        xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
        xmlns:umb="urn:umbraco.library"
        exclude-result-prefixes="umb">

        <xsl:output method="xml" indent="yes" omit-xml-declaration="yes" />

        <xsl:param name="currentPage" />
        <xsl:variable name="siteRoot" select="$currentPage/ancestor-or-self::*[@level = 2]" />
      
          
        <xsl:template match="/">
                <xsl:if test="$siteRoot/*[@isDoc][not(umbracoNaviHide = 1)]">
                        <ul class="sidebarInnerTopMenuNavSub">
                                
                                <!-- Start with children of the site root -->
                                <xsl:apply-templates select="$siteRoot/*[@isDoc]" />
                        </ul>
                </xsl:if>
        </xsl:template>
          

        <xsl:template match="*[@isDoc]">
                <li>
                        <a href="{umb:NiceUrl(@id)}">
                                <!-- Check if $currentPage is below or at this level -->
                                <xsl:if test="descendant-or-self::*[@id = $currentPage/@id]">
                                        <xsl:attribute name="class">current</xsl:attribute>
                                </xsl:if>
                                <xsl:value-of select="@nodeName" />
                        </a>
                        
                        <!-- If currentPage is below or at this level AND there is at least one visible child -->
                        <xsl:if test="descendant-or-self::*[@id = $currentPage/@id] and *[@isDoc][not(umbracoNaviHide = 1)]">
                                <ul>
                                        <xsl:apply-templates select="*[@isDoc]" />
                                </ul>
                        </xsl:if>
                </li>
        </xsl:template>

        <!-- This automatically hides items with umbracoNaviHide checked -->
        <xsl:template match="*[umbracoNaviHide = 1]" />

</xsl:stylesheet>