记录层次--改变收集答案时的代码

function Print(pRoot)
{
    // write code here
    let res = []
    if(!pRoot) return res;
    let q = [[pRoot,0]];
    while(q.length){
        const [n,l] = q.shift();
            if(!res[l]){
            res.push([n.val]);
            }else{
                if(l % 2 === 0 ){
                    res[l].push(n.val);
                }else{
                    res[l].unshift(n.val);
                }
            }
        if(n.left) q.push([n.left,l+1]);
        if(n.right) q.push([n.right,l+1]);
        }
    return res;
    }

一层一层的访问，有条件的改变答案的插入

（这里不是改变队列中节点的插入，而是答案收集时的插入）

function Print(pRoot)
{
    // write code here
    let res = []
    if(!pRoot) return res;
    let q = [pRoot];
    let l = 0;
    while(q.length){
        let len = q.length;
        res.push([]);
        l++;
        while(len--){
            const n = q.shift();
            //答案收集时的有条件地更改
            if(l % 2 == 0){
                res[res.length - 1].unshift(n.val);
            }else{
                res[res.length - 1].push(n.val);
            }
            if(n.left) q.push(n.left);
            if(n.right) q.push(n.right);
        }
    }
    return res;
}