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:

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

# 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