题解 | #按之字形顺序打印二叉树#

public class Solution {
    public ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<>(new ArrayList<>());
        if (pRoot == null) return res;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(pRoot);
        TreeNode temp;
        boolean flag = false; // 隔行反转
        while (!queue.isEmpty()) {
            ArrayList<Integer> arr = new ArrayList<>();
            int len = queue.size();
            for (int i = 0; i < len; i++) {
                temp = queue.poll();
                arr.add(temp.val);
                if (temp.left != null) queue.add(temp.left);
                if (temp.right != null) queue.add(temp.right);
            }
            if (flag) Collections.reverse(arr);
            flag = !flag;
            res.add(arr);
        }
        return res;
    }
}