Count Subarrays of Length Three With a Condition

Count Subarrays of Length Three With a Condition

What is this problem about?

The "Count Subarrays of Length Three With a Condition interview question" is a fixed-size window problem. You are given an array of integers and need to count how many contiguous subarrays of length exactly three satisfy a specific condition (for example, the first and third elements are equal, or the sum matches a target). This is an introductory array challenge focused on window traversal.

Why is this asked in interviews?

Microsoft and Meta ask the "Count Subarrays of Length Three coding problem" to evaluate a candidate's basic loop control and array indexing. It’s a warm-up question that ensures you can handle boundaries (not going out of index) and correctly extract segments of a specific length. It checks for clean, concise coding in the "Array interview pattern" category.

Algorithmic pattern used

This problem follows the Sliding Window (Fixed Size) pattern.

1. Linear Loop: Iterate through the array from index 0 up to $n - 3$.
2. Window Extraction: For each index $i$, consider the elements at $i, i+1$, and $i+2$.
3. Condition Check: Apply the logic required by the problem (e.g., arr[i] == arr[i+2]).
4. Count: Increment a counter for every match found.

Example explanation

Array: [1, 2, 1, 3, 1], Condition: arr[i] == arr[i+2]

• i=0: [1, 2, 1]. $1 == 1$. (Count = 1)
• i=1: [2, 1, 3]. $2 eq 3$.
• i=2: [1, 3, 1]. $1 == 1$. (Count = 2) Result: 2.

Common mistakes candidates make

• Index Out of Bounds: Forgetting to stop the loop at $n-3$, causing the code to look for $i+2$ beyond the array end.
• Confusion with Subsequences: Treating the problem as if the elements don't need to be adjacent.
• Incorrect Length: Using a window of size 2 or 4 instead of 3.

Interview preparation tip

When a problem specifies a small fixed length (like 3), a simple loop with direct index access is often better and faster to write than a general-purpose sliding window implementation. Keep it simple and focus on boundary correctness.