Microsoft OA Questions Practical Sharing | Detailed explanation of two Microsoft OA questions

Preparing Microsoft In the process of writing oa questions, many students will have an illusion: the question seems not difficult, but when it comes to actually writing it, either the boundary conditions cannot be passed, or the problem is overturned on the hidden use cases.

The 26th Microsoft OA this time is very typical - neither question is "difficult to calculate", but both strongly test whether you understand the question and whether you can quickly come up with ideas. The following is a complete breakdown of the timeline, real question ideas, and common misunderstandings based on real test experience, giving students who are brushing up on Microsoft OA a more practical reference.

Microsoft interview timeline

OA received approximately 1–2 weeks after delivery

10/3 Received oa

10/16 Received final round

10/26 final round three rounds

11/8 The recruiter sent an email saying large volume of recruiting

11/21 action center complete

12/11 offer

Microsoft OA real question sharing

Question 1: Maximize the MEX of the array

Given an array of integers, you can operate on any element to reduce it to any integer in the interval [0, original value]. The goal is to maximize the MEX (minimum missing non-negative integer) of the entire array by adjusting the array and return this maximum MEX.

Require

For MEX = m, there must be at least one each of 0,1,2,…,m-1 in the array
Each number can only be "decreased", not made larger
So, large numbers can "fill the hole", but if the decimals are too small, it will be hopeless.

Greedy ideas

First sort the array from small to large
Maintain a current number x that "needs to be made up", initially 0
Traverse the sorted array from left to right:
- If the current element ≥ x, it means that it can be reduced to x to "occupy a pit"
- After successfully occupying the pit, x++, continue to find the next one
- If the current element < x, it means that this number is no longer helpful and skip it directly.
After the traversal is completed, the current x is the maximum MEX that can be obtained

Question 2: String rolling encryption (Roll operation)

Given a string s, and an array roll. Each roll[i] means: do a letter cycle +1 (a→b,...,z→a) for the first roll[i] characters of s. Execute all roll[i] in order and output the final string.

Brute force solution

If each roll[i] really changes the first roll[i] characters
The worst case is O(n²), which will definitely be TLE

The optimization idea is to use differential arrays:

Don't care about "which operation", only care about "how many times each position has been added in total"
Record interval addition using difference arrays

Specific practices

Initialize a difference array diff of length n, all zeros
For each roll[i]:
- Diff[0] += 1
- If roll[i] < n, then diff[roll[i]] -= 1
Do a prefix sum on diff to get the total number of times each position is added cnt[i]
For each character s[i]:
- Calculate (s[i] – ‘a’ + cnt[i]) % 26
- Convert back to characters

The time complexity is O(n + m) and the space complexity is O(n).

FAQ-Microsoft OA Questions

Q1: Does Microsoft OA focus more on algorithms or implementation?
A: Watch both. The first question focuses on the correctness of the idea, and the second question obviously tests whether you are familiar with differences/prefixes and optimization techniques commonly used in this kind of engineering.

Q2: How many questions can LeetCode stably solve for this set of questions?
A: The question itself is not difficult, but if you have never seen MEX greedy or interval difference, it is easy to be confused in OA. In addition to reviewing the questions, it is more important to summarize the "question templates".

Q3: Will Python suffer?
A: No, as long as the idea is right and the complexity is stable, Python is completely sufficient. The real disadvantage is that I wrote about violence without realizing it.

Why Microsoft OA isn’t the “hard problem” that really drives the gap

On the whole, this time Microsoft OA is more like screening candidates for "whether the foundation is solid and whether the ideas are clear", rather than trying to solve the problem. If you are often stuck at an inflection point in your thinking in OA, or know the direction but the on-site implementation is unstable, the difference will be very obvious if you understand the high-frequency model thoroughly in advance, or if you have someone to help you correct the direction at key nodes.

If you find yourself in these situations often in OA or technical situations——

The general direction is right, but the details become more and more confusing.
I know what algorithm to use, but I’m always one step behind.
I tend to get nervous during the interview and my performance is significantly lower than usual.

If you truly "understand" high-frequency question types and common solutions in advance, or have someone help you correct your direction at key nodes, the improvement will be very obvious.

If you plan to pursue the OA/technical aspects of Microsoft or other major North American manufacturers in the future, we will also provide long-term support here. OA assistance , VO assistance, interview Q&A and other support, from idea dismantling to implementation details, all are done personally, and more importantly, to help you minimize on-the-spot instability problems.