Hierarchical traversal of binary tree

Give you a binary tree , Please return to its button Sequence traversal The resulting node value . （ That is, layer by layer , Access all nodes from left to right ）.

Example ：
Binary tree ：[3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
   
   
  Return its sequence traversal result ：

[
  [3],
  [9,20],
  [15,7]
]

This is a question on Li Kou , But I didn't do it , So I understand the official practice .

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> ret = new ArrayList<List<Integer>>();
        if(root==null){
            return ret;
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        // Root in line 
        queue.offer(root);
        while(!queue.isEmpty()){
            List<Integer> level = new ArrayList<Integer>();
            int currentLevelSize = queue.size();
            // Every time the parent node leaves the queue , Add its child nodes to the queue 
            for(int i=1;i<=currentLevelSize;++i){
                // Out of line operation , Get elements from the head of the team 
                TreeNode node = queue.poll();
                level.add(node.val);
                if(node.left!=null){
                    // The left child is not empty. The left child joins the team 
                    queue.offer(node.left);
                }
                if(node.right!=null){
                    // The right child is not empty , The right kid joins the team 
                    queue.offer(node.right);
                }
            }
            ret.add(level);
        }
        return ret;
    }
}

The main idea of the official practice is ,

1. Root in line
2. Traverse the current queue , Dequeue nodes
3. When the node leaves the queue , Add its child nodes to the queue , Ensure that the queue is not empty after each traversal
4. When the queue is empty , Represents that all traversals have been completed