题目链接：按之字型顺序打印二叉树

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 {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     *
     * @param pRoot TreeNode类
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> Print (TreeNode root) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();

        if(root == null) return res;

        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        int i = 1;
        while(q.size() > 0)
        {
            ArrayList<Integer> lev = new ArrayList<>();
            int len = q.size();
            for(int j = 0; j < len; j ++)
            {
                TreeNode t = q.peek();
                q.poll();
                lev.add(t.val);
                if(t.left != null) q.offer(t.left);
                if(t.right != null) q.offer(t.right);
            }
            if(i % 2 == 0) Collections.reverse(lev);
            res.add(lev);
            i ++;
        }
        return res;
    }
}