# 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;
}

## Output

If you run the above code, then you will get the following result.

54426

## Conclusion

If you have any queries in the tutorial, mention them in the comment section.