Find Special Substring of Length K

Find Special Substring of Length K

What is this problem about?

The Find Special Substring of Length K interview question focuses on pattern recognition and contiguous segments in a string. A "special" substring is defined as a contiguous part of the string of length exactly k where every character is identical. Furthermore, to be truly special, the characters immediately before and after this segment (if they exist) must be different from the character within the segment. You need to determine if such a substring exists.

Why is this asked in interviews?

Amazon and other tech firms use the Find Special Substring of Length K coding problem to assess a candidate's string manipulation skills and attention to boundary conditions. It tests whether you can accurately implement a set of multi-part rules: length constraint, uniformity constraint, and isolation constraint. It evaluations your ability to write clean, bug-free loops.

Algorithmic pattern used

This problem typically uses the Sliding Window interview pattern or a Linear Scan with Counters.

1. Iterate: Traverse the string while keeping track of current consecutive identical characters.
2. Length Check: When a segment of identical characters ends (or the string ends), check if its length is exactly k.
3. Isolation Check: Since we only trigger the check when the character changes, the "isolation" from the right is naturally handled. You simply need to ensure the length is exactly k and not greater.

Example explanation

Suppose s = "aaabaaa" and k = 3.

1. We see "aaa". Its length is 3. The character after it is 'b'. 'b' != 'a'.
2. This segment is special! We return true. If s = "aaaa" and k = 3:
3. The only segment of identical characters is "aaaa", which has length 4.
4. Even though it contains a substring of length 3, that substring would be flanked by an 'a', which violates the isolation rule.
5. Result: False.

Common mistakes candidates make

• Over-counting: Identifying "aaa" inside "aaaa" as a valid substring of length 3. The rule usually implies the entire block of identical characters must be size k.
• Index Out of Bounds: Failing to check if the character at i-1 or i+k exists before comparing it.
• String Slicing: Repeatedly creating new substring objects in a loop, which can lead to $O(N \cdot K)$ complexity instead of $O(N)$.

Interview preparation tip

When dealing with "segments of identical items," always keep a running count. Reset the count to 1 whenever the character changes. This avoids the need for complex nested loops and keeps your "String interview pattern" code efficient.