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
        return visit(pRoot);
    }

    public ArrayList<ArrayList<Integer>> visit(TreeNode root) {
        if (root == null) {
            return new ArrayList<>();
        }
        ArrayList<ArrayList<Integer>> ans = new ArrayList<>();

        ArrayDeque<TreeNode> queue = new ArrayDeque<>();
        queue.add(root);
        ArrayDeque<TreeNode> nextqueue = new ArrayDeque<>();

        ArrayList<Integer> level = new ArrayList<>();
        while(!queue.isEmpty()) {
            TreeNode top = queue.poll();
            if(top == null) {
                continue;
            }

            
            level.add(top.val);
            if (top.left != null) {
            nextqueue.add(top.left);

            }
            if (top.right != null) {
            nextqueue.add(top.right);

            }
           
            
             if (queue.isEmpty()) {
                queue = nextqueue.clone();
                ans.add(level);
                level = new ArrayList<>();
                nextqueue.clear();
            }
        }

        for (int i = 0;i<ans.size();i++) {
            if (i%2 !=0) {
                ans.set(i,reverse(ans.get(i)));
            }
        }

        return ans;

    }

    public ArrayList<Integer> reverse(ArrayList<Integer> nums) {
        ArrayList<Integer> rev = new ArrayList<>();
        for (int i = nums.size()-1;i>=0;i--) {
            rev.add(nums.get(i));
        }
        return rev;
    }
}