331. Verify Preorder Serialization of a Binary Tree

Description

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.

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 ','.

Solution

verify-preorder-serialization-of-a-binary-tree.py

class Solution:
    def isValidSerialization(self, preorder: str) -> bool:
        preorder = preorder.split(',')
        diff = 1

        for node in preorder:
            diff -= 1
            if diff < 0: return False
            if node != '#':
                diff += 2

        return diff == 0