原生JS实现点击展开与折叠二级菜单

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<head> 
 <meta charset="UTF-8"> 
 <title>Document</title> 
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  ul li:hover{ 
   cursor: pointer; 
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</head> 
<body> 
 <ul> 
  <li onclick = "f('sub_menu_1')">一级菜单1 
   <ul id="sub_menu_1"> 
    <li>二级餐单1</li> 
    <li>二级餐单1</li> 
    <li>二级餐单1</li> 
    <li>二级餐单1</li> 
   </ul> 
  </li> 
  <li onclick="f('sub_menu_2')">一级菜单2 
   <ul id="sub_menu_2"> 
    <li>二级菜单2</li> 
    <li>二级菜单2</li> 
    <li>二级菜单2</li> 
    <li>二级菜单2</li> 
   </ul> 
 
  </li> 
  <li>一级餐单3</li> 
 </ul> 
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    if(dis_v == "block") 
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    var sub_menu = document.getElementById(str); 
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    if(dis_v == "none") 
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