题解 | #按之字形顺序打印二叉树#使用两个辅助栈

public ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {

        if (pRoot == null){
            return new ArrayList<>();
        }

        ArrayList<ArrayList<Integer>> list = new ArrayList<>();
        // 从左往右
        Stack<TreeNode> first = new Stack<>();
        // 从右往左
        Stack<TreeNode> second = new Stack<>();
        first.push(pRoot);
        int flag = 1;

        while (!first.isEmpty() || !second.isEmpty()){

            ArrayList<Integer> curList = new ArrayList<>();

            if (flag % 2 != 0){

                   while (!first.isEmpty()){

                       TreeNode curNode = first.pop();
                       curList.add(curNode.val);

                       if (curNode.left != null){
                           second.push(curNode.left);
                       }

                       if (curNode.right != null){
                           second.push(curNode.right);
                       }
                   }

            }else {

                while (!second.isEmpty()){

                    TreeNode curNode = second.pop();
                    curList.add(curNode.val);

                    if (curNode.right != null){
                        first.push(curNode.right);
                    }

                    if (curNode.left != null){
                        first.push(curNode.left);
                    }
                }
            }

            flag++;
            list.add(curList);
        }
        return list;
    }