题目：

题解：

class Solution:
    def isValidSerialization(self, preorder: str) -> bool:
        pre = 1
        for i in preorder.split(','):
            if i.isdigit():
                if pre == 0:
                    return False
                pre += 1
            else:
                if pre == 0:
                    return False
                pre -= 1
        return pre == 0