题解 | #按之字形顺序打印二叉树#

import java.util.*;
import java.util.ArrayList;

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

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

    }

}
*/
//层次遍历，记录下每层节点，奇数层不变，偶数层翻转
public class Solution {
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        if(pRoot==null) return new ArrayList();
        Queue<TreeNode>queue=new LinkedList<>();
        ArrayList<ArrayList<Integer>>result=new ArrayList<>();
        queue.offer(pRoot);
        int curlevel=1;
        //层次遍历，入队
        while(!queue.isEmpty()){
            int size=queue.size();
            ArrayList<Integer>temp=new ArrayList<>();
            
            for(int i=0;i<size;i++){
                TreeNode cur=queue.poll();
                temp.add(cur.val);
                if(cur.left!=null)queue.offer(cur.left);
                if(cur.right!=null)queue.offer(cur.right);
            }
            //偶数层翻转
            if(curlevel%2==0){
                for(int i=0;i<size/2;i++){
                    int tmp=temp.get(i);
                    temp.set(i,temp.get(size-1-i));
                    temp.set(size-1-i,tmp);
                }
            }
            curlevel++; 
            result.add(temp);
        }
        return result;
    }
}