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
     *
     *分层遍历，然后根据行数，顺序或者倒序装入集合，返回结果。
     * @param pRoot TreeNode类
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> Print (TreeNode pRoot) {
        // write code here
        ArrayList<ArrayList<Integer>> result = new ArrayList<>();
        if (pRoot == null)
            return result;
        Deque<TreeNode> deque = new LinkedList<>();
        deque.offer(pRoot);
        

        int lineNum = 0;
        while (!deque.isEmpty()) {
            ArrayList<Integer> levelList = new ArrayList<>();
            int n = deque.size();
            for (int i = 0; i < n; i++) {
                TreeNode node = deque.poll();
                levelList.add(node.val);

                if (node.left != null) {
                        deque.add(node.left);
                    }
                    if (node.right != null) {
                        deque.add(node.right);
                    }
            }
            if (lineNum % 2 == 0){
                result.add(levelList);
            }else{
                Collections.reverse(levelList);
                result.add(levelList);
            }
            lineNum++;
        }
        return result;
    }
}