整个看下来还是感觉迷迷糊糊的

按照层次遍历的顺序存储

按照从上到下，从左到右的顺序树存，一般二叉树会增添一些空节点形成完全二叉树

import java.util.Queue;
import java.util.LinkedList;
class TreeNode{
    public int val;
    public TreeNode left;
    public TreeNode right;
    public TreeNode(int val) {
        this.val = val;
        this.left = null;
        this.right = null;
    }
}
 
public class Main {
    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);
        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(3);
        TreeNode node4 = new TreeNode(4);
        TreeNode node5 = new TreeNode(5);
        TreeNode node6 = new TreeNode(6);
        TreeNode node7 = new TreeNode(7);
        root.left = node2;
        root.right = node3;
        node2.left = node4;
        node2.right = node5;
        node3.left = node6;
        node3.right = node7;
         
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
         
        while(!queue.isEmpty()) {
            //根
            TreeNode temp = queue.poll();
            System.out.printf("%d ", temp.val);
            //左
            if(temp.left != null) {
                queue.offer(temp.left);
            }
            //右
            if(temp.right != null) {
                queue.offer(temp.right);
            }
        }
    }
}
 
//打印结果：
1 2 3 4 5 6 7