题解 | #按之字形顺序打印二叉树 在层序遍历的基础上加个判断和反转即可#

import java.util.*;

/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        if (pRoot == null){
            return new ArrayList<>();
        }
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        boolean flag = true;
        queue.add(pRoot);
        while (!queue.isEmpty()){
            int length = queue.size();
            flag = flag == true ? false : true;
            ArrayList<Integer> nums = new ArrayList<>();
            for (int i = 0; i < length; i++) {
                TreeNode tmpNode = queue.poll();
                if (tmpNode.left != null){
                    queue.add(tmpNode.left);
                }
                if (tmpNode.right != null){
                    queue.add(tmpNode.right);
                }
                nums.add(tmpNode.val);
            }
            if (flag){
                Collections.reverse(nums);
            }
            res.add(nums);
        }
        return res;
    }

}