104. Maximum Depth of Binary Tree

class Solution:
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return 0

        count = 0
        q = collections.deque()
        q.append(root)
        while q:
            count += 1
            num = len(q)
            for i in range(num):
                node = q.popleft()
                if node.left:
                    q.append(node.left)
                if node.right:
                    q.append(node.right)


        return count