剑指Offer：按之字形顺序打印二叉树

题目描述

请实现一个函数按照之字形打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右至左的顺序打印，第三行按照从左到右的顺序打印，其他行以此类推。

解题思路

package com.geekerstar.s60;

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.Queue;

public class Solution {
    public ArrayList> Print(TreeNode pRoot) {
        ArrayList> ret = new ArrayList<>();
        Queue queue = new LinkedList<>();
        queue.add(pRoot);
        boolean reverse = false;
        while (!queue.isEmpty()) {
            ArrayList list = new ArrayList<>();
            int cnt = queue.size();
            while (cnt-- > 0) {
                TreeNode node = queue.poll();
                if (node == null)
                    continue;
                list.add(node.val);
                queue.add(node.left);
                queue.add(node.right);
            }
            if (reverse)
                Collections.reverse(list);
            reverse = !reverse;
            if (list.size() != 0)
                ret.add(list);
        }
        return ret;
    }

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

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

        }

    }
}