# 331 Verify Preorder Serialization of a Binary Tree

One way to serialize a binary tree is to use preorder traversal. When we encounter a non-null node, we record the node's value. If it is a null node, we record using a sentinel value such as `'#'`.

``````    9
/ \
3   2
/ \   \
4   1   6
``````

For example, the above binary tree can be serialized to the string `"9,3,4,#,#,1,#,#,2,#,6,#,#"`, where `'#'` represents a null node.

Given a string of comma-separated values `preorder`, return `true` if it is a correct preorder traversal serialization of a binary tree.

It is guaranteed that each comma-separated value in the string must be either an integer or a character `'#'` representing null pointer.

You may assume that the input format is always valid.

• For example, it could never contain two consecutive commas, such as `"1,,3"`.

Note: You are not allowed to reconstruct the tree.

Example 1:

``````Input: preorder = "9,3,4,#,#,1,#,#,2,#,6,#,#"
Output: true
``````

Example 2:

``````Input: preorder = "1,#"
Output: false
``````

Example 3:

``````Input: preorder = "9,#,#,1"
Output: false
``````

Constraints:

• `1 <= preorder.length <= 104`
• `preorder` consist of integers in the range `[0, 100]` and `'#'` separated by commas `','`.
 `````` 1 2 3 4 5 6 7 8 9 10 11 12 13 `````` ``````class Solution: def isValidSerialization(self, preorder: str) -> bool: slots = 1 for node in preorder.split(','): slots -= 1 if slots < 0: return False if node != '#': slots += 2 return slots == 0``````