按之字形顺序打印二叉树

思路：使用两个栈，从stackOut弹出结点并添加到当前层，将该结点的孩子结点按照下一轮打印顺序压入stackIn。

    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        if(pRoot==null) return new ArrayList<>();
        ArrayList<ArrayList<Integer>> answer=new ArrayList<>();
        Stack<TreeNode> stack1=new Stack<>();
        Stack<TreeNode> stack2=new Stack<>();
        //两个栈与打印方向
        Stack<TreeNode> stackOut=null;
        Stack<TreeNode> stackIn=null;
        boolean dirctionFlag=false;//false代表下一轮从右向左打印
        //tmp val
        TreeNode node=null;

        stack1.push(pRoot);
        while(!(stack1.empty()&&stack2.empty())){
            ArrayList<Integer> currentFloor=new ArrayList<>();
            stackOut=stack1.empty()?stack2:stack1;
            stackIn=stack1.empty()?stack1:stack2;
            //一轮打印
            while(!stackOut.empty()){
                node=stackOut.pop();
                currentFloor.add(node.val);
                if(dirctionFlag) {
                    if(node.right!=null) stackIn.push(node.right);
                    if(node.left!=null) stackIn.push(node.left);
                }
                else{
                    if(node.left!=null) stackIn.push(node.left);
                    if(node.right!=null) stackIn.push(node.right);
                }
            }
            answer.add(currentFloor);
            dirctionFlag=!dirctionFlag;
        }
        return answer;
    }