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) {
        // write code here
        // 解题思路：层序遍历时间复杂度要求O(n)，可以采用队列来实现
        if (pRoot == null) {
            return new ArrayList();
        }

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(pRoot);
        ArrayList<ArrayList<Integer>> arrayList = new ArrayList<>();
        boolean resever = true;

        while (!queue.isEmpty()) {
            ArrayList<Integer> list = new ArrayList<>();
            int l = queue.size();
            resever = !resever;

            for (int i = 0; i < l; i++) {
                TreeNode node = queue.poll();
                list.add(node.val);

                if (node.left != null) {
                    queue.offer(node.left);
                }

                if (node.right != null) {
                    queue.offer(node.right);
                }
            }

            if (resever) {
                Collections.reverse(list);
            }
            arrayList.add(list);
        }

        return arrayList;
    }
}