1、實現原理：獲取所有行數，利用“取餘”原理進行css顏色賦值

2、整體程式碼：

<script>
  window.onload = function (){
    var lis = document.getElementsByTagName("li");
    for( var i=0; i<lis.length; i++){
        if(i%2 == 0 ){
            lis[i].style.backgroundColor = "#eee";
        }

        lis[i].onmounseover = function (){
            this.className = "current";
        }
        
        lis[i].onmouseout = function(){
            this.className = "";
        }
        
    }
  }
</script>