题解 | #按之字形顺序打印二叉树#

1. 层序遍历，队列存储元素

2. 标记奇偶层，隔1层做1次数组反转

3. 两层循环：层内循环结束，进入下一层的循环

    public ArrayList<ArrayList<Integer>> Print(TreeNode pRoot){
    ArrayList<ArrayList<Integer>> outArr = new ArrayList<>();
    if(pRoot==null)return outArr;//不能返回null
    Queue<TreeNode> q = new LinkedList<>();
    q.offer(pRoot);
    int level = 1;
    while(!q.isEmpty()){
    int size = q.size();
    ArrayList<Integer> inArr = new ArrayList<>();
    for(int i=0;i<size;i++){
        TreeNode cur = q.poll();
        if(cur.left!=null)q.offer(cur.left);
        if(cur.right!=null)q.offer(cur.right);
        inArr.add(cur.val);
    }
    if(level%2==0){
        Collections.reverse(inArr);
        outArr.add(inArr);
    }else{
        outArr.add(inArr);
    }
    level++;
 }
        return outArr;
}