Validate As Before Bs | Dataford Interview Questions

phData pipeline validation uses compact marker strings containing uppercase A, uppercase B, and optional separator characters such as -, _, or spaces. Return True if every A occurs before every B in the string. Characters other than A and B do not affect the ordering.

Once a B has been observed, encountering an A makes the string invalid. The function should process the input in one left-to-right pass and may stop as soon as the ordering is violated.

Formal Specification

Implement ordered_markers(s), where s is a string. Return a Boolean:

True if no A appears after any B.
False otherwise.

An empty string or a string containing only separators, only A characters, or only B characters is valid.

Constraints

0 <= len(s) <= 10^6
s contains only uppercase A, uppercase B, spaces, hyphens, and underscores
The input must not be modified
Use O(1) auxiliary space

Function Signature

def ordered_markers(s):

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Problem