Monotone Increasing Digits (LC 738)

Return the largest number not exceeding n whose digits are monotonically non-decreasing. The greedy fix is to find the first descending pair, drop that digit by one, and fill the rest with nines. Requirements Given a non-negative integer n, return the largest number ≤ n whose digits are monotonically increasing (each digit ≤ the next). Greedy approach: scan digits left to right, and at the first position where a digit exceeds its successor, decrement it and set every digit after it to 9. Notes Equivalent to LeetCode 738. Reported as the second problem in an onsite coding round, immediately after the parity-swap problem. The greedy insight (find the breaking pair, drop by one, fill nines) is the part candidates stall on; deriving it on a small example such as 332 first makes it click.