An integer has monotone increasing digits if and only if each pair of adjacent digits `x` and `y` satisfy `x <= y`.

Given an integer `n`, return the largest number that is less than or equal to `n` with monotone increasing digits.

Example 1:

    Input: n = 10
    Output: 9

Example 2:

    Input: n = 1234
    Output: 1234

Example 3:

    Input: n = 332
    Output: 299

Constraints:

- `0 <= n <= 109`