Monotone Increasing Digits

Given a non-negative integer n, return the largest integer x such that x <= n and the decimal digits of x are monotone increasing. A number has monotone increasing digits if every digit is less than or equal to the next digit from left to right, i.e. for every adjacent pair a[i] and a[i+1], a[i] <= a[i+1]. In the interview, the original warm-up may have been a “no digit replacement” version, such as checking whether a number is already monotone increasing. The follow-up corresponds to LeetCode 738, where you need to construct the largest valid x. Input An integer n Output An integer representing the largest monotone increasing number less than or equal to n. Constraints 0 <= n <= 10^9 Examples Example 1 Input: 10 Output: 9 Example 2 Input: 1234 Output: 1234 Example 3 Input: 332 Output: 299 Example Input 10 Output 9