把二叉树打印成多行

思路：做这题前先看看从上往下打印二叉树

import java.util.*;
public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
         ArrayList<Integer> list = new ArrayList<Integer>();
        if(root==null)return list;

         Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while(!queue.isEmpty()){
            TreeNode temp = queue.poll();
            list.add(temp.val);//给list复制还是要用.val
            if(temp.left!=null)queue.offer(temp.left);
            if(temp.right!=null)queue.offer(temp.right);
        }
        return list;
    }
}

然后发现这题就是在从上往下打印基础上多了个层次问题。在while中加个int size = queue.size();就vans了

import java.util.*;
public class Solution {
    ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer>> thelist = new ArrayList<ArrayList<Integer>>();
        if(pRoot==null)return thelist; //这里要求返回thelist而不是null
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(pRoot);
        while(!queue.isEmpty()){
            ArrayList<Integer> list = new ArrayList<Integer>();
            int size = queue.size();
            for(int i=0;i<size;i++){
            TreeNode temp = queue.poll();
            list.add(temp.val);
            if(temp.left!=null)queue.offer(temp.left);
            if(temp.right!=null)queue.offer(temp.right);
            }
            thelist.add(list);
        }
        return thelist;
    }

}