LC102. Binary Tree Level Order Traversal

# 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 []
 
        res = []
        q = deque([root])
 
        while q:
            level = []
 
            for _ in range(len(q)):
                node = q.popleft()
                level.append(node.val)
 
                if node.left:
                    q.append(node.left)
                
                if node.right:
                    q.append(node.right)
            
            res.append(list(level))
 
        return list(res)

Complexity

Time: $O(n)$
Space: $O(n)$