← JP Morgan Interview Insights
My first instinct was to sort both halves and compare, which works but I fumbled explaining the complexity.
First, clarify the problem and edge cases, then propose an efficient solution using frequency counting. For each half, count character frequencies and compute the number of mismatches, which equals the minimum changes needed.
Pro tip: Mention that the answer is simply the number of characters in the left half that are not matched in the right half, and that this can be computed in O(n) time with O(1) space (since alphabet size is constant).
Restate the problem in your own words and confirm that the digit string has even length. Ask clarifying questions if needed.
Recognize that two strings are anagrams if their character frequency counts are identical. The minimum changes needed is the number of characters in one half that are not balanced by the other half.
Count frequencies of digits in the left half and right half. For each digit, compute the absolute difference in counts and sum them; the answer is half of that sum (or equivalently, the number of excess characters in the left half).
State that the algorithm runs in O(n) time and O(1) space (since there are only 10 possible digits).
Walk through a simple example, such as '1234', to verify the approach and handle edge cases like all characters already matching.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.