题解 | #按之字形顺序打印二叉树#

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 pRoot) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        // write code here
        if (pRoot == null) {
            return res;
        }

        Queue<TreeNode> queue = new ArrayDeque<TreeNode>();
        queue.offer(pRoot);

        int level = 0;
        while (!queue.isEmpty()) {
            ArrayList<Integer> lineNode = new ArrayList<Integer>();
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (node == null) {
                    continue;
                }
                lineNode.add(node.val);
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            
            if (level % 2 != 0) { // 从右向左时，反序下行数据
                Collections.reverse(lineNode);
            }
            res.add(lineNode);
            level++;
        }
        return res;
    }
}