Check if Binary String Has at Most One Segment of Ones

Check if Binary String Has at Most One Segment of Ones

What is this problem about?

The "Check if Binary String Has at Most One Segment of Ones interview question" is a string property verification. You are given a binary string $s$ that does not have leading zeros. You need to determine if there is at most one contiguous block of '1's in the string. For example, "1100" has one segment, while "1101" has two segments.

Why is this asked in interviews?

Cisco and other networking companies use the "Check if Binary String Has at Most One Segment coding problem" to assess basic string parsing skills. It evaluates whether a candidate can detect transitions between characters. It’s an introductory "String interview pattern" question that tests for simple but robust logic.

Algorithmic pattern used

This problem is solved using a Single Pass Transition Count.

1. Search for Gap: Because the string has no leading zeros, it starts with a segment of ones.
2. The "01" Rule: If you ever encounter the sequence "01" in the string, it means a segment of ones started after a segment of zeros had already appeared. This implies there are at least two segments of ones.
3. Validation: If the substring "01" is found, return false. Otherwise, return true.

Example explanation

String: "11000"

• No "01" found. The ones are all at the beginning. Result: True. String: "1001"
• "01" is found at index 2-3. This means a new segment of ones started after the first one ended. Result: False.

Common mistakes candidates make

• Over-counting: Trying to count the total number of '1's instead of the number of segments.
• Ignoring Constraints: Forgetting that the string is guaranteed to start with '1', which simplifies the check to just searching for "01".
• Inefficient Splits: Splitting the string by '0' and counting non-empty parts. While correct, searching for "01" is faster and uses less memory.

Interview preparation tip

For binary string problems, always look for the "forbidden transition." Often, a property is violated only by a specific character sequence (like "01" or "101"). Identifying these sequences is a key "String interview pattern" skill.