> ## Documentation Index
> Fetch the complete documentation index at: https://leetcode-py.wisl.dev/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> leetcode-py is a Python LeetCode practice environment generator with one CLI: lcpy. It is not a service or platform.
> Each problem is a directory under leetcode/ with README.md, solution.py, test_solution.py, helpers.py, and playground.ipynb. lcpy gen creates them from JSON templates bundled with the package.
> Examples are backed by tests; copy them verbatim.

# Verify Preorder Serialization of a Binary Tree

> Tested Python solution for LeetCode 331 with 18 pytest cases. Generate a practice environment with lcpy.

LeetCode 331, [Medium](/catalog/medium). Topics: [String](/catalog/topics/string), [Stack](/catalog/topics/stack), [Tree](/catalog/topics/tree), [Binary Tree](/catalog/topics/binary-tree). [View on LeetCode](https://leetcode.com/problems/verify-preorder-serialization-of-a-binary-tree/description/).

Generate this problem as a practice environment: tested reference solution, 18 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 331   # by problem number
lcpy gen -s verify_preorder_serialization_of_a_binary_tree   # by problem name
```

## Problem

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 `#`.

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.

### Examples

![Example 1](https://assets.leetcode.com/uploads/2021/03/12/pre-tree.jpg)

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

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

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

### Constraints

* 1 \<= preorder.length \<= 10^4
* preorder consist of integers in the range \[0, 100] and '#' separated by commas ','.

**Note:** You are not allowed to reconstruct the tree.

## Solution

Reference implementation from [solution.py on GitHub](https://github.com/wislertt/leetcode-py/blob/main/leetcode/verify_preorder_serialization_of_a_binary_tree/solution.py), full suite in [test\_solution.py](https://github.com/wislertt/leetcode-py/blob/main/leetcode/verify_preorder_serialization_of_a_binary_tree/test_solution.py):

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(n)
    # Space: O(1)
    def is_valid_serialization(self, preorder: str) -> bool:
        slots = 1
        for token in preorder.split(","):
            if slots <= 0:
                return False
            if token == "#":
                slots -= 1
            else:
                slots += 1
        return slots == 0
```

## Complexity

| Time | Space |
| - | - |
| O(n) | O(1) |

## Tags


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