Monotone Increasing Digits in C++

Suppose we have a non-negative integer N, we have to find the largest number that is less than or equal to N with monotone increasing digits. We know that an integer has monotone increasing digits if and only if each pair of adjacent digits’ x and y satisfy x <= y.) So if the input is like 332, then the result will be 299.

To solve this, we will follow these steps −

• s := N as string, i := 1, n := size of s
• while i < n and s[i] >= s[i – 1]
  • increase i by 1
• if i < n
  • while i > 0 and s[i – 1] > s[i], then
    • decrease i by 1
    • decrease s[i] by 1
• for j in range i + 1 to n
  • s[j] := ‘9’
• return s as number

Let us see the following implementation to get better understanding −

Example

 Live Demo

#include <bits/stdc++.h>
using namespace std;
class Solution {
public:
   int monotoneIncreasingDigits(int N) {
      string s = to_string(N);
      int i = 1;
      int n = s.size();
      while(i < n && s[i] >= s[i - 1]) i++;
      if( i < n)
      while(i > 0 && s[i - 1] > s[i]){
         i--;
         s[i]--;
      }
      for(int j = i + 1; j < n; j++)s[j] = '9';
      return stoi(s);
   }
};
main(){
   Solution ob;
   cout << (ob.monotoneIncreasingDigits(332));
}

Input

332

Output

299