思路：遞迴，擴容

增加一個deep引數，內部list數量與deep相同，每個深度對應一個list

根節點新增到list(deep-1),然後分別對其左右子樹遞迴求每層的節點，深度每到一個孩子+1

import java.util.ArrayList;


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

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

    }

}
*/
public class
 
 Solution {
    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
        ArrayList<ArrayList<Integer>> list=new ArrayList<>();
        depth(pRoot,1,list);
        return list;
    }
        public void depth(TreeNode root,int deep,ArrayList<ArrayList<Integer>> list){
        
 
if(root==null){
            return;
        }
        if(deep>list.size()){
            list.add(new ArrayList<>());
        }
        list.get(deep-1).add(root.val);
        depth(root.left,deep+1,list);
        depth(root.right,deep+1,list);
    }
    
}