← JP Morgan Interview Insights
My first instinct was greedy: always pick the largest available character.
Use a greedy strategy with a max-heap to always pick the largest available character that doesn't violate the repeat limit. Track the last used character and its consecutive count, and if the largest character is blocked, temporarily use the next largest character to break the sequence.
Pro tip: Clarify that you don't have to use all characters, so you can stop when no valid character can be placed. Also, mention that this greedy approach is optimal because always choosing the largest possible character maximizes lexicographic order at each step.
Restate the problem: build the lexicographically largest string where no character appears more than repeatLimit times consecutively, and you may skip characters. Note that the input string can be rearranged, and characters not used are simply ignored.
Count the frequency of each character and push all characters with positive counts into a max-heap (priority queue) ordered by character value. This allows efficient retrieval of the largest available character.
While the heap is not empty, pop the largest character. If it equals the last used character and its consecutive count has reached repeatLimit, pop the next largest character instead, append it, and push the blocked character back. Otherwise, append the largest character and update its consecutive count.
If the heap becomes empty or the only available character is blocked, stop. Also, if after using a character its count becomes zero, do not push it back. Ensure that when you use a different character, the consecutive count for the previous character resets.
State that the time complexity is O(n log k) where n is the total number of characters used and k is the number of distinct characters (at most 26 for lowercase letters). Walk through a small example to demonstrate correctness.
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