# 102. Binary Tree Level Order Traversal

#### Medium

***

Given the `root` of a binary tree, return *the level order traversal of its nodes' values*. (i.e., from left to right, level by level).

 

**Example 1:**

![](https://assets.leetcode.com/uploads/2021/02/19/tree1.jpg)

```
Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]
```

**Example 2:**

```
Input: root = [1]
Output: [[1]]
```

**Example 3:**

```
Input: root = []
Output: []
```

 

**Constraints:**

* The number of nodes in the tree is in the range `[0, 2000]`.
* `-1000 <= Node.val <= 1000`

```python
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        if not root:
            return []
        q = [root, "$"]
        levels = []
        current_level = []
        while q:
            node = q.pop(0)
            if node == "$":
                if len(q) != 0:
                    q.append("$")
                levels.append(current_level)
                current_level = []
                continue
            current_level.append(node.val)
            if node.left:
                q.append(node.left)
            if node.right:
                q.append(node.right)
        return levels
```