Find the closest and smaller tidy number in C++

Suppose we have a number n, we have to find the closest and smaller tidy number of n. So a number is called tidy number, if all of its digits are sorted in non-decreasing order. So if the number is 45000, then the nearest and smaller tidy number will be 44999.

To solve this problem, we will traverse the number from end, when the tidy property is violated, then we reduce digit by 1, and make all subsequent digit as 9.

Example

Live Demo

#include<iostream>
using namespace std;
string tidyNum(string number) {
   for (int i = number.length()-2; i >= 0; i--) {
      if (number[i] > number[i+1]) {
         number[i]--;
         for (int j=i+1; j<number.length(); j++)
            number[j] = '9';
      }
   }
   return number;
}
int main() {
   string str = "45000";
   string num = tidyNum(str);
   cout << "The tidy number is: " << num;
}