In this article, we will dive into various ways to reverse a number using integer reversal. We will begin by exploring the iterative approach, which breaks down numbers into individual digits and rebuilds them in reverse order. Next, we will discuss recursion as an alternative solution. We will also use a built-in function `Math.Pow()`

for number reversal. Additionally, we will explore the two-pointer approach, where elements from opposite ends are swapped until they meet. Finally, we will examine the advantages of using the `BigInteger`

class to handle large numbers.

Let’s start.

## Create a Benchmark Baseline

Let’s set up our code environment to explore integer reversal techniques. We’ll utilize a console application and create separate methods for each reversal technique, each accepting an integer parameter `num`

and returning the reversed integer `reversedNumber`

.

But before we tackle different integer methods, we need to establish a baseline for string reversal. We’ll use it later to compare performance between integer and string reversal techniques.

Let’s create a method `ReverseAsString()`

which accepts an integer parameter `num`

and returns the reversed number:

public static int ReverseAsString(int num) { if (num == int.MaxValue || num == int.MinValue || num == 0) { return 0; } var isNegative = num < 0; ReadOnlySpan<char> numChars = Math.Abs(num).ToString().AsSpan(); var length = numChars.Length; Span<char> reversedChars = stackalloc char[length]; for (int i = 0; i < length; i++) { reversedChars[i] = numChars[length - i - 1]; } if (int.TryParse(reversedChars, out int reversedNumber)) { return isNegative ? -reversedNumber : reversedNumber; } else { return 0; } }

Here, we convert the `num`

parameter into a character span `reversedChars`

. Then we iterate through `reversedChars`

and store each character in the reversed order. Finally, we convert this character array into an integer `reversedNumber`

.

This approach provides a straightforward and efficient method for reversing integers using string manipulation techniques.

With the string-based reversal baseline done, let’s delve into reversing integers directly.

**Reverse a Number by Digit Extraction and Reconstruction**

This approach delves into the number, extracts each digit, and then places them in a temporary variable. The extracted digits are rearranged and reassembled to form the reversed number.

Let’s create a method `ReverseUsingDigitExtractionAndReconstruction()`

to implement this approach:

public static int ReverseUsingDigitExtractionAndReconstruction(int num) { int maxValueDiv10 = int.MaxValue / 10; int minValueDiv10 = int.MinValue / 10; if (num == int.MaxValue || num == int.MinValue || num == 0) { return 0; } var reversedNumber = 0; while (num != 0) { var remainder = num % 10; if (reversedNumber > maxValueDiv10 || reversedNumber < minValueDiv10) { return 0; } reversedNumber = reversedNumber * 10 + remainder; num /= 10; } return reversedNumber; }

Our `ReverseUsingDigitExtractionAndReconstruction()`

method reverses an integer by extracting and rebuilding its digits. To start, we initialize `reversedNumber`

to zero. Inside our `while`

loop, we extract the last digit of `num`

, multiply the current `reversedNumber`

by 10 to shift digits left, and add the extracted digit.

Finally, we divide `num`

by 10 to remove the last digit. This loop repeats until all digits are processed, resulting in the reversed integer in `reversedNumber`

.

## Recursion to Reverse a Number

In recursion, a function calls itself within its definition, unlike traditional loops with counters. The recursion continues iteratively until the pre-defined base case is satisfied, triggering the unwinding of the call stack.

To understand this better, we use the same steps as `ReverseUsingDigitExtractionAndReconstruction()`

method, but instead of a `while`

loop, we use recursion.

Let’s declare a method `ReverseUsingRecursion()`

with two parameters – an integer `num`

and an optional `reversedNumber`

which defaults to zero:

public static int ReverseUsingRecursion(int num, int reversedNumber = 0) { int maxValueDiv10 = int.MaxValue / 10; int minValueDiv10 = int.MinValue / 10; if (num == int.MaxValue || num == int.MinValue || num == 0) { return reversedNumber; } if (reversedNumber > maxValueDiv10 || reversedNumber < minValueDiv10) { return 0; } var remainder = num % 10; reversedNumber = reversedNumber * 10 + remainder; return ReverseUsingRecursion(num / 10, reversedNumber); }

We extract the last digit of `num`

and add it to a left-shifted `reversedNumber`

(multiplied by 10) to build the reversed integer digit by digit. To process the remaining digits, we call `ReverseUsingRecursion()`

again with the remaining number, `num/10`

and the updated `reversedNumber`

. This chain of calls constructs the reversed number digit by digit. Finally, the last call returns the complete reversed number, propagating it back to the initial call for the final result.

## Math.Pow

In .NET `Math.Pow()`

is a built-in method in the Math class, that replaces repetitive multiplication with efficient exponentiation.

Let’s create a method `ReverseUsingMathPow()`

that reverses integer `num`

using `Math.Pow()`

:

public static int ReverseUsingMathPow(int num) { int maxValueDiv10 = int.MaxValue / 10; var isNegative = num < 0; num = isNegative ? -num : num; if (num == int.MaxValue || num == int.MinValue || num == 0) { return 0; } int length = (int)Math.Floor(Math.Log10(num) + 1); var reversedNumber = 0; var powersOf10 = new int[10]; powersOf10[0] = 1; for (int i = 1; i < 10; i++) { powersOf10[i] = powersOf10[i - 1] * 10; } for (int i = length - 1; i >= 0; i--) { var remainder = num % 10; if ((remainder * powersOf10[i] / 10) >= maxValueDiv10) { return 0; } reversedNumber += remainder * powersOf10[i]; num /= 10; } return isNegative ? -reversedNumber : reversedNumber; }

Within our `while`

loop, we extract the rightmost digit `remainder`

using `num % 10`

. The `remainder`

is then multiplied by the current power of 10 `powerOf10`

and added to the `reversedNumber`

. Finally, we remove the processed digit from `num`

and update the position value for the next iteration, continuing until all digits are reversed.

Here we use `Math.Log10()`

to calculate the number of digits in `num`

, however, it cannot process negative numbers. Therefore, to handle negative numbers during reversal, we convert them to positive ones before applying these techniques and then restore the original sign afterward.

Let’s transition from right-to-left digit extraction to a bidirectional approach to reverse a number.

## Reverse a Number With Digit Swapping

This time, we can employ an iterative approach, traversing the digits of a number from both ends, gradually progressing toward the center.

Let’s create `ReverseBySwappingDigits()`

to reverse number `num`

and return the reversed version `reversedNumber`

:

public static int ReverseBySwappingDigits(int num) { int maxValueDiv10 = int.MaxValue / 10; if (num == int.MaxValue || num == int.MinValue || num == 0) { return 0; } var isNegative = num < 0; num = isNegative ? -num : num; int reversedNumber = 0; int totalNumOfDigits = (int)Math.Floor(Math.Log10(num)) + 1; int[] powersOf10 = new int[10]; powersOf10[0] = 1; for (int i = 1; i < 10; i++) { powersOf10[i] = powersOf10[i - 1] * 10; } var leftPowIndex = totalNumOfDigits - 1; var rightPowIndex = 0; while (leftPowIndex >= rightPowIndex) { var leftDigit = (num / powersOf10[leftPowIndex]) % 10; var rightDigit = (num / powersOf10[rightPowIndex]) % 10; if ((rightDigit * (powersOf10[leftPowIndex] / 10)) >= maxValueDiv10) { return 0; } if (leftPowIndex != rightPowIndex) { reversedNumber += leftDigit * powersOf10[rightPowIndex]; reversedNumber += rightDigit * powersOf10[leftPowIndex]; } else { reversedNumber += leftDigit * powersOf10[leftPowIndex]; } leftPowIndex--; rightPowIndex++; } return isNegative ? -reversedNumber : reversedNumber; }

To begin, we set up `isNegative`

, `reversedNumber`

, and `totalNumOfDigits`

to manage the original number’s sign, reversed version, and digit count. The `leftPowIndex`

and `rightPowIndex`

variables track corresponding powers of 10 for the leftmost and rightmost digits.

Starting from both ends, we iterate through digit pairs using temporary variables `leftDigit`

and `rightDigit`

. We swap their positions by leveraging their respective powers of 10 and adding them to `reversedNumber`

.

After each iteration `leftPowIndex`

shifts right and `rightPowIndex`

shifts left, effectively moving towards the middle digit. This continues until the loop finishes or surpasses the middle (for odd-numbered digits). The final result is the reversed integer.

We’ve explored different ways to reverse fixed-length integers, now let’s see how to reverse large numbers.

## Reversing Large Numbers

While fixed-length data types like `int`

handle integer reversal for smaller values, their susceptibility to overflow and underflow necessitates alternative approaches for larger numbers. We leverage the **BigInteger data type, which empowers us to handle significantly larger numbers effortlessly.**

Let’s dive into the specifics of customizing the **digit extraction and reconstruction** approach to handle large number reversal:

public static BigInteger ReverseUsingDigitExtractionAndReconstruction(BigInteger num) { BigInteger reversedNumber = 0; while (num != 0) { var remainder = num % 10; reversedNumber = reversedNumber * 10 + remainder; num /= 10; } return reversedNumber; }

Here. we declare the `ReverseUsingDigitExtractionAndReconstruction()`

method, which takes the `BigInteger`

parameter `num`

. The remaining logic remains the same as we had for `int`

datatype, except that we have removed the checks for overflow and underflow conditions.

`BigInteger`

is a powerful tool for working with large integers, but it does have some disadvantages to consider. **Due to their complexity, BigInteger operations are generally slower than working with native integer data types like int or long.** This can be noticeable for frequent operations on large numbers. It also requires more memory to store as compared to primitive types.

We’ve explored different techniques to reverse a number as an integer, but choosing the optimal approach depends on real-world performance. Benchmarking helps us quantify the efficiency of each number reversal technique, guiding the selection for our specific needs.

## Benchmarking Number Reversal Techniques

Benchmarks measure the execution time and memory usage of different code sections, helping identify the most efficient approaches. But, before we dive into our benchmark results, let’s familiarize ourselves with what each column represents.

Alright, now we’re ready to explore the benchmark results. For our benchmark, we set the `ReverseAsString()`

method as the baseline, which represents the standard way of doing things.

Let’s benchmark all the integer reversal approaches for `int`

datatype numbers:

| Method | Mean | Ratio | RatioSD | Gen0 | Allocated | |------------------------------- |----------:|------:|--------:|-------:|----------:| | Recursion_Int | 20.18 ns | 0.12 | 0.02 | - | - | | DigitExtractAndReconstruct_Int | 20.38 ns | 0.12 | 0.02 | - | - | | MathPow_Int | 48.29 ns | 0.30 | 0.04 | 0.0153 | 64 B | | SwappingDigits_Int | 52.11 ns | 0.32 | 0.04 | 0.0153 | 64 B | | ReverseAsString_Int | 166.77 ns | 1.00 | 0.00 | 0.0553 | 232 B |

We can see that the best approach to reversing a `int`

type number is `ReverseUsingDigitExtractionAndReconstruction()`

. It is due to its simplicity, reliance on efficient integer arithmetic, and avoidance of complex string operations. Our other approaches involve intricate mathematical operations that introduce additional overhead hence impacting performance.

Now, it’s time to see how our reversal techniques perform with large numbers. Let’s run our benchmark on a large 200-digit number:

| Method | Mean | Ratio | RatioSD | Gen0 | Allocated | |------------------------------------------ |-----------:|------:|--------:|--------:|----------:| | ReverseAsString_200DigitNumber | 2.733 us | 1.00 | 0.00 | 0.6752 | 2.76 KB | | DigitExtractAndReconstruct_200DigitNumber | 45.359 us | 16.56 | 1.50 | 9.7046 | 39.85 KB | | Recursion_200DigitNumber | 54.964 us | 20.13 | 3.36 | 9.7046 | 39.85 KB | | MathPow_200DigitNumber | 74.215 us | 27.20 | 5.14 | 14.8926 | 61.21 KB | | SwappingDigits_200DigitNumber | 131.924 us | 48.45 | 6.14 | 14.8926 | 61.35 KB |

For large numbers, such as those with 200 digits, reversing them as `string`

often outperforms reversing them as `BigInteger`

.

## Conclusion

In this article, we explored various approaches to reverse a number as an integer. After that, we provided different benchmark results for each approach using small and big numbers for testing. This provided quite an interesting conclusion about which approaches are the most performant.