ul li多层包含时，怎么不让子级撑开父级

Question

<div id="main">
  
   <div class="tree">
    <ul>
        <li>
            <a href="#">一代</a>
            <ul>
                <li>
                    <a href="#">二代</a>
                    <ul>
                        <li><a href="#">三代</a></li>
                    </ul>
                </li>
                <li>
                    <a href="#">二代</a>
                    <ul>
                        <li>
                            <a href="#">三代</a>
                            
                        </li>
                        <li>
                            <a href="#">三代</a>
                            
                        </li>
                        <li><a href="#">三代</a></li>
                    </ul>
                </li>
                <li><a href="#">二代</a></li>
                <li><a href="#">二代</a></li>
            </ul>
        </li>
    </ul>
</div>
  
</div>

* {margin: 0; padding: 0;}
.tree{width:1200px; margin:40px auto 0 auto}
.tree ul {
    padding-top: 20px; position: relative;
    
    transition: all 0.5s;
    -webkit-transition: all 0.5s;
    -moz-transition: all 0.5s;
}

.tree li {
    float: left; text-align: center;
    list-style-type: none;
    position: relative;
    padding: 20px 5px 0 5px;
    
    transition: all 0.5s;
    -webkit-transition: all 0.5s;
    -moz-transition: all 0.5s;
    
}

/*We will use ::before and ::after to draw the connectors*/

.tree li::before, .tree li::after{
    content: '';
    position: absolute; top: 0; right: 50%;
    border-top: 1px solid #ccc;
    width: 50%; height: 20px;
}
.tree li::after{
    right: auto; left: 50%;
    border-left: 1px solid #ccc;
}

/*We need to remove left-right connectors from elements without 
any siblings*/
.tree li:only-child::after, .tree li:only-child::before {
    display: none;
}

/*Remove space from the top of single children*/
.tree li:only-child{ padding-top: 0;}

/*Remove left connector from first child and 
right connector from last child*/
.tree li:first-child::before, .tree li:last-child::after{
    border: 0 none;
}
/*Adding back the vertical connector to the last nodes*/
.tree li:last-child::before{
    border-right: 1px solid #ccc;
    border-radius: 0 5px 0 0;
    -webkit-border-radius: 0 5px 0 0;
    -moz-border-radius: 0 5px 0 0;
}
.tree li:first-child::after{
    border-radius: 5px 0 0 0;
    -webkit-border-radius: 5px 0 0 0;
    -moz-border-radius: 5px 0 0 0;
}

/*Time to add downward connectors from parents*/
.tree ul ul::before{
    content: '';
    position: absolute; top: 0; left: 50%;
    border-left: 1px solid #ccc;
    width: 0; height: 20px;
}

.tree li a{
    border: 1px solid #ccc;
    padding: 5px 10px;
    text-decoration: none;
    color: #666;
    font-family: arial, verdana, tahoma;
    font-size: 11px;
    display: inline-block;
    
    border-radius: 5px;
    -webkit-border-radius: 5px;
    -moz-border-radius: 5px;
    
    transition: all 0.5s;
    -webkit-transition: all 0.5s;
    -moz-transition: all 0.5s;
}

.tree li a h3{
    display: block;
    margin-top: 10px;
    max-width:150px;
}

.tree li a span{
    display: block;
    margin-top: 10px;
    max-width:150px;
}

.tree li a img{
    width: 150px;
    height: 200px;
    object-fit: cover;
    margin-top: 10px;
}

.tree li a p{
    max-width:150px;
    margin-top: 10px;
}

/*Time for some hover effects*/
/*We will apply the hover effect the the lineage of the element also*/
.tree li a:hover, .tree li a:hover+ul li a {
    background: #c8e4f8; color: #000; border: 1px solid #94a0b4;
}
/*Connector styles on hover*/
.tree li a:hover+ul li::after, 
.tree li a:hover+ul li::before, 
.tree li a:hover+ul::before, 
.tree li a:hover+ul ul::before{
    border-color:  #94a0b4;
}

Answer 1

<div class="a">
  <p class="t">一级</p>
  <ul class="b">
    <li class="c">
      <p class="t">二级</p>
      <ul class="b">
        <li class="c">
          <p class="t">三级</p>
        </li>
        <li class="c">
          <p class="t">三级</p>
        </li>
        <li class="c">
          <p class="t">三级</p>
        </li>
        <li class="c">
          <p class="t">三级</p>
        </li>
        <li class="c">
          <p class="t">三级</p>
        </li>
        <li class="c">
          <p class="t">三级</p>
        </li>
        <li class="c">
          <p class="t">三级</p>
        </li>
        <li class="c">
          <p class="t">三级</p>
        </li>
        <li class="c">
          <p class="t">三级</p>
        </li>
      </ul>
    </li>
    <li class="c">
      <p class="t">二级</p>
      <ul class="b">
        <li class="c"></li>
      </ul>
    </li>
  </ul>
</div>

*{
  margin:0;
  padding:0;
  list-style:none;
  text-align:center;
}
.a{
  width:400px;
  background:yellow;
}
.b{
  display:-webkit-box
}
.c{
  -webkit-box-flex:1;
  width:0;
}

Answer 2