Largest smaller number possible using only one swap operation in C++

In this tutorial, we are going to write a program that finds the largest number with a single swap that is less than the given number n.

Let's see the steps to solve the problem.

Initialise the number n.
Iterate from the end of the string and find the index of the digit which is greater than its next digit. Store it in a variable.
Break the loop as soon as u find it.
Iterate over the number from the end of the string to the above index.
- Find the index of the digit which is less the above indexed digit and is greater among all in the area.
Swap the digits at the above two indexes. Return the updated number.

Example

Let's see the code.

Live Demo

#include <bits/stdc++.h>
using namespace std;
string getTheNumber(string str) {
   int length = str.length();
   int index = -1;
   for (int i = length - 2; i >= 0; i--) {
      if (str[i] > str[i+1]) {
         index = i;
         break;
      }
   }
   int smallerDigitIndex = -1;
   for (int i = length - 1; i > index; i--) {
      if (str[i] < str[index]) {
         if (smallerDigitIndex == -1 || str[i] >= str[smallerDigitIndex]) {
            smallerDigitIndex = i;
         }
      }
   }
   if (index == -1) {
      return "-1";
   }
   if (smallerDigitIndex != -1) {
      swap(str[index], str[smallerDigitIndex]);
      return str;
   }
   return "-1";
}
int main() {
   string str = "54624";
   cout << getTheNumber(str) << endl;
   return 0;
}