//第一种
var aLi = document.getElementsByTagName("li");
for(let i=0;i<aLi.length;i++){
aLi[i].onclick = function(){
for(var j=0;j<aLi.length;j++){
aLi[j].style.background = "white";
}
aLi[i].style.background = "red";
}
}
//第二种
$("li").click(function(){
$(this).css("background","red").siblings().css("background","white");
})