First clarify the problem and edge cases, then propose a greedy algorithm that scans digits from left to right to find the first violation of monotonicity. When a violation is found, decrement the digit at the violation point, set all subsequent digits to 9, and backtrack to fix any new violations. Finally, handle leading zeros and return the result.
Pro tip: Mention that you can avoid string conversion by using arithmetic operations, but string manipulation is often clearer and less error-prone in an interview. Also, test with edge cases like n=0, n=10, and n=1234 to ensure correctness.
Confirm that digits are compared from left to right (most significant to least) and that monotone increasing means each digit is <= the next. Discuss edge cases such as n < 10, n with repeated digits, and n = 0.
Scan the digits from left to right to find the first position where a digit is greater than the next digit. If no violation, n itself is the answer.
At the violation, decrement the offending digit, set all following digits to 9, and then backtrack to ensure the prefix remains monotone increasing. Repeat if necessary.
After adjustments, remove any leading zeros that may have been introduced (e.g., if the first digit becomes 0).
Convert the resulting digit array back to an integer and return it. Walk through a few examples to verify correctness.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.