按之字顺序打印二叉树

链接：https://www.nowcoder.com/questionTerminal/91b69814117f4e8097390d107d2efbe0?f=discussion
来源：牛客网

public static ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        int layer = 1;
        //s1存奇数层节点
        Stack<TreeNode> s1 = new Stack<TreeNode>();
        s1.push(pRoot);
        //s2存偶数层节点
        Stack<TreeNode> s2 = new Stack<TreeNode>();

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

        while (!s1.empty() || !s2.empty()) {
            if (layer%2 != 0) {
                ArrayList<Integer> temp = new ArrayList<Integer>();
                while (!s1.empty()) {
                    TreeNode node = s1.pop();
                    if(node != null) {
                        temp.add(node.val);
                        System.out.print(node.val + " ");
                        s2.push(node.left);
                        s2.push(node.right);
                    }
                }
                if (!temp.isEmpty()) {
                    list.add(temp);
                    layer++;
                    System.out.println();
                }
            } else {
                ArrayList<Integer> temp = new ArrayList<Integer>();
                while (!s2.empty()) {
                    TreeNode node = s2.pop();
                    if(node != null) {
                        temp.add(node.val);
                        System.out.print(node.val + " ");
                        s1.push(node.right);
                        s1.push(node.left);
                    }
                }
                if (!temp.isEmpty()) {
                    list.add(temp);
                    layer++;
                    System.out.println();
                }
            }
        }
        return list;
    }