题解 | #NC14 按之字形顺序打印二叉树#

第一层直接输出，第二层反转输出，以此类推，奇数层直接输出，偶数层反转

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 {
    public ArrayList<ArrayList<Integer>> Print(TreeNode root) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        if (root != null) queue.offer(root);
        int layer = 1;

        while (!queue.isEmpty()) {
            int n = queue.size();
            ArrayList<Integer> list = new ArrayList<>();
            for (int i = 0; i < n; 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 (layer % 2 == 1) res.add(new ArrayList<>(list));
            else {
                Collections.reverse(list);
                res.add(list);
            }
            layer++;
        }
        return res;
    }

}