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http://ife.baidu.com/course/d...
I am doing a question from Baidu Front-end Academy, the link is as above. The topic is to implement a binary tree traversal. The js code is as follows:

//綁定變量
var root=document.getElementById("root");
var btn1=document.getElementById("btn1");
var btn2=document.getElementById("btn2");
var btn3=document.getElementById("btn3");
var timer=0;
//顯示節(jié)點
function showNode(node){
    //node.style.backgroundColor = 'red';
    alert(timer);
    setTimeout(function(){
        node.style.backgroundColor="red";
    },timer+=100);
    setTimeout(function(){
        node.style.backgroundColor="#ffffff";
    },timer+=100);
}
//先序遍歷
function preOrder(node){
    if(!(node == null)){
        showNode(node);
        preOrder(node.children[0]);
        preOrder(node.children[1]);
    }
}
//使用遞歸方式實現(xiàn)中序遍歷
function inOrder(node){
    if(!(node == null)){
        //alert("btn2");
        inOrder(node.children[0]);//先訪問左子樹
        showNode(node);//再訪問根節(jié)點
        inOrder(node.children[1]);//最后訪問右子樹
    }
}
//后序遍歷
function postOrder(node){
    if(!(node == null)){
        postOrder(node.children[0]);
        postOrder(node.children[1]);
        showNode(node);
    }
}
//綁定事件
btn1.onclick=function(){
    preOrder(root);
    timer=0;
}
btn2.onclick=function(){
    inOrder(root);
    timer=0;
}
btn3.onclick=function(){
    postOrder(root);
    timer=0;
}

There is no error in the code, but I just can’t understand why timer =100 is used for the time in setTimeout?
Why can’t I use 100 directly?
I am puzzled. Front-end newbie, please give me some advice!