102. Binary Tree Level Order Traversal

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
import collections

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root:
            return []

        # queue FIFO
        q = collections.deque()
        q.append(root)              # q = [ TreeNode(3) ]

        res = []
        while q:
            num = len(q)
            level = []
            for _ in range(num):
                node = q.popleft()
                level.append(node.val)

                if node.left:
                    q.append(node.left)
                if node.right:
                    q.append(node.right)

            res.append(level)

        return res