# 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]]:

        res = []

        q = collections.deque()

        # add the root to the queue as the base case
        if root: q.append(root)

        # run outer loop till no elements left
        # ensures been through all levels
        while q:
            # num elements at a single level
            qLen = len(q)
            level = []

            # iterate over elements of a single level in one pass
            for i in range(qLen):

                # pop the FIFO element of the queue and append to the result of level
                node = q.popleft()
                level.append(node.val)

                # add to the queue only if there is a child node
                if node.left:
                    q.append(node.left)

                # add to the queue only if there is a child node
                if node.right:
                    q.append(node.right)

            # append results of the single level
            res.append(level)

        return res