<p 
      v-for="(item, index) in 20"
      :key="'a'+index"
      :
      @click="addClass(item, index)">{{item}}</p>
//css
.plist{
      width: 100%;
      height: 40px;
      border-radius: 10px;
      list-style: none;
      text-align: center;
      line-height: 40px;
      font-size: 30px;
    }
    .plist:hover {
      background-color: #2887f0;
      color: #fff;
    }
    .active {
      background-color: #2887f0;
      color: #fff;
    }
//js
data(){
      return {
       activeClass: -1
     }
}
addClass(item, index){
     this.activeClass = index
 }