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Posted on Jun 14

Tesla OA Questions & Solutions | SQL, String Manipulation and Prefix Sum Problems

I recently completed the Tesla Software Engineer Online Assessment and managed to AC all three questions in about 30 minutes. The problem set covered three very different areas: SQL, string processing, and prefix-sum hashing. None of the questions were extremely difficult, but each one had a few subtle edge cases that could easily cost you hidden test cases if you weren't careful.

Here's a breakdown of the three questions along with the key ideas and common pitfalls.

Question 1: SQL – Test Group Statistics

The first question involved two tables:

test_groups – stores group names and point values for each test case in the group.
test_cases – stores execution results for individual test cases.

The task was to calculate, for every test group:

Total number of test cases
Number of passed test cases
Total score

The final result needed to be sorted by total score descending and group name ascending.

Solution Idea

Use test_groups as the primary table and perform a LEFT JOIN with test_cases. Then aggregate by group name.

Common Pitfalls

The biggest trap was using an INNER JOIN instead of a LEFT JOIN.

If a group contains no test cases, an INNER JOIN removes the group entirely from the result set. However, the problem explicitly requires every group to appear, with a score of 0 if no test cases exist.

Another subtle issue is score calculation. The correct formula is:

Passed Test Cases × Test Value

Many candidates mistakenly use:

SUM(test_value)

Since test_value exists at the group level, failed test case rows still contain the same value. Summing directly will overcount and produce incorrect scores.

A good validation test is creating a group with no associated test cases and verifying that it still appears in the final output with a score of zero.

Question 2: Insert Maximum Number of 'a' Characters

Given a string S, insert as many lowercase 'a' characters as possible at arbitrary positions.

The constraint is that the final string must never contain three consecutive 'a' characters.

If the original string already contains "aaa", return -1.

Solution Idea

Treat every non-'a' character as a separator.

These separators divide the string into multiple gaps. Each gap can contain at most two consecutive 'a' characters. If more than two appear within a gap, the string would contain "aaa".

For every gap:

Remaining Capacity = max(0, 2 - Existing A Count)

The answer is simply the sum of the remaining capacities across all gaps.

Common Pitfalls

The first mistake is forgetting the final gap after the last character. If the calculation only occurs when a non-'a' character is encountered, the last segment is never processed.

The second mistake is misunderstanding the limit. Each gap may contain at most two total 'a' characters, not two newly inserted characters.

For example:

S = "aa"

The gap already contains two 'a's, so its remaining capacity is 0. The correct answer is 0, not 2.

Question 3: Count Zero-Sum Subarrays

Given an integer array A, count the number of subarrays whose sum equals zero.

If the answer exceeds 1,000,000,000, return -1.

Solution Idea

This is a classic Prefix Sum + Hash Map problem.

Initialize:

prefixSum = 0
result = 0
map = {0:1}

As you iterate through the array:

Update prefix sum.
Check how many times the current prefix sum has appeared before.
Add that frequency to the result.
Update the hash map.

Whenever the same prefix sum appears twice, the subarray between those positions has sum zero.

Common Pitfalls

The order of operations matters:

Check frequency
→ Add to result
→ Update frequency

Changing this order can easily lead to overcounting or missing valid subarrays.

Another important detail is the initialization:

{0:1}

Without it, any zero-sum subarray starting from index 0 will be missed.

The overflow condition should also be checked immediately after each update to the result rather than waiting until the end.

Consider:

A = [2, -2, 3, 0, 4, -7]

Prefix sums become:

2, 0, 3, 3, 7, 0

The final answer is 4, corresponding to four valid zero-sum subarrays.

Final Thoughts

Overall, the Tesla OA felt very manageable if you're comfortable with common interview patterns. None of the questions required advanced algorithms, but each one contained subtle implementation details that could easily cause hidden test case failures.

The SQL question tested careful aggregation logic, the string problem focused heavily on edge-case handling, and the prefix-sum question rewarded candidates who understood the standard hash-map counting pattern.

Interview Preparation Support

Before taking the OA, I worked with Interview Aid to review common Tesla-style assessment patterns and edge cases. Having someone point out the typical hidden pitfalls ahead of time saved a lot of debugging time during the actual assessment.

Interview Aid provides: