Given a non-negative integer N, find the largest number that is less than or
equal to N with monotone increasing digits.
(Recall that an integer has monotone increasing digits if and only if each pair of
adjacent digits x and y satisfy x <= y.)
Example 1:
Input: N = 10 Output: 9
Example 2:
Input: N = 1234 Output: 1234
Example 3:
Input: N = 332 Output: 299
Note:
N is an integer in the range [0, 10^9].