331. Verify Preorder Serialization of a Binary Tree

class Solution(object):
    def isValidSerialization(self, preorder):
        """
        :type preorder: str
        :rtype: bool
        """
        #a null node use a slot 
        #a non-null node use a slot but create 2 slots, net gain 1 slot 
        nodes=preorder.split(',')
        slot=1
        for node in nodes:
            if slot==0:return False
            if node=='#':
                slot-=1
            else:
                slot+=1
                
        return slot==0