Rubrik·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Rubrik SWE interview with a camera-on coding round. One problem, pretty involved, and I'm still not 100% sure I nailed the edge cases. The constraint about every suffix having balanced character frequencies is the kind of thing that sounds manageable until you actually try to build a valid arrangement.

Questions Asked (1)

Given a string S and an integer K, determine whether you can rearrange the characters of S such that for every suffix of the rearranged string, the frequency difference between any two distinct characters in S is at most K. If a valid rearrangement exists, return the lexicographically smallest one; otherwise return -1.

Algorithms & Data Structures

Author's notes

The suffix constraint is what makes this nasty.

Create a free account to read the full note

AI HintsAI Generated

Suggested Approach

First, check feasibility by analyzing the maximum frequency and the number of distinct characters: a valid arrangement exists iff maxFreq - 1 <= K * (numDistinct - 1). Then, construct the lexicographically smallest string greedily by trying characters in sorted order and checking if the remaining characters can still satisfy the suffix condition, using a priority queue or frequency counts to maintain balance.

Pro tip: Clarify the definition of 'suffix' early—whether it includes the entire string or only proper suffixes—as it affects the feasibility condition. Also, mention that the greedy choice can be validated in O(1) with a precomputed feasibility check, avoiding expensive simulations.

1. Clarify the problem

Confirm the exact meaning of 'suffix' and 'frequency difference' with the interviewer. Ensure you understand that for every suffix (including the full string), the difference between the highest and lowest frequency among all distinct characters in S is at most K.

2. Derive feasibility condition

Let n be the length, d the number of distinct characters, and f_max the maximum frequency. Show that a valid arrangement exists if and only if f_max - 1 <= K * (d - 1). Explain why this condition is necessary and sufficient.

3. Design greedy construction

Build the result from left to right. At each position, try characters in lexicographical order. For each candidate, temporarily decrement its frequency and check if the remaining multiset can still satisfy the suffix condition (using the feasibility condition on the remaining counts). Choose the first valid character.

4. Implement efficient check

Maintain the frequencies and the current maximum frequency. After choosing a character, update the maximum frequency if needed. The feasibility check for the remaining string can be done in O(1) by comparing the new maximum frequency with K and the number of distinct characters with non-zero frequency.

5. Analyze complexity and edge cases

The greedy approach runs in O(n * d) time, which is acceptable for typical constraints. Discuss edge cases: K=0, all characters same, multiple characters with same frequency, and when no valid arrangement exists.

Key Points to Mention

Feasibility condition: maxFreq - 1 <= K * (numDistinct - 1)
Greedy lexicographical construction with backtracking check
Efficient O(1) feasibility check using frequency counts
Handling of edge cases: K=0, single distinct character, empty string
Time and space complexity analysis
Proof of correctness for the greedy choice

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.