How to Find Reverse of a Number in Java
Reversing a number in Java is a common task that can be accomplished using a variety of methods. For example, you can use a loop to iterate over the digits of the number and build the reversed number one digit at a time. Alternatively, you can convert the number to a string, reverse the string using the built-in StringBuilder class, and then parse the reversed string back into a number. Other options include using a character array or recursion to reverse the number. When deciding on the best method, consider factors such as time complexity, space complexity, and code simplicity.
In this blog we will learn 5 different ways in which you can write a function to find the reverse of a number in Java.
Problem: Write a Java program that reads an integer from the user and prints the reverse of the integer. The program should continue to prompt the user for integers until the user enters 0.
Example:
Enter a number: 12345 The reverse of 12345 is 54321 Enter a number: 123 The reverse of 123 is 321 Enter a number: 0 Cannot reverse a single digit number
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Method 1: Reverse a Number in Java Using a Loop
- Initialize a variable reverse to 0.
- While num is not 0:
- Append the last digit of num to reverse by performing the following steps:
- Multiply reverse by 10.
- Add the last digit of num to reverse.
- Remove the last digit of num by dividing num by 10.
- Return reverse.
public static int reverse(int num) { int reverse = 0; while (num != 0) { reverse = reverse * 10 + num % 10; num /= 10; } return reverse;}
Dry Run Explanation:
Let’s say we want to reverse the number 12345. Here is how the method would work:
| Iteration | reverse | num | num % 10 | New value of reverse | New value of num |
|---|---|---|---|---|---|
| 1 | 0 | 12345 | 5 | 5 | 1234 |
| 2 | 5 | 1234 | 4 | 54 | 123 |
| 3 | 54 | 123 | 3 | 543 | 12 |
| 4 | 543 | 12 | 2 | 5432 | 1 |
| 5 | 5432 | 1 | 1 | 54321 | 0 |
Read more: Reverse a String in Java
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Method 2: Reverse a Number in Java Using a String
- Convert num to a string.
- Initialize a variable reverse to an empty string.
- Iterate through the characters of the string in reverse order:
- Append each character to reverse.
- Convert reverse back to an integer and return it.
public static int reverse(int num) { String str = Integer.toString(num); String reverse = ""; for (int i = str.length() - 1; i >= 0; i--) { reverse += str.charAt(i); } return Integer.parseInt(reverse);}
Dry Run Explanation:
Let’s say we want to reverse the number 12345. Here is how the method would work:
| Iteration | i | str.charAt(i) | reverse | New value of reverse |
|---|---|---|---|---|
| 1 | 4 | 5 | “” | “5” |
| 2 | 3 | 4 | “5” | “54” |
| 3 | 2 | 3 | “54” | “543” |
| 4 | 1 | 2 | “543” | “5432” |
| 5 | 0 | 1 | “5432” | “54321” |
Method 3: Reverse a Number in Java Using an Array
- Convert num to a string and then to an array of characters.
- Initialize an array reverse of the same size as the original array.
- Iterate through the original array in reverse order:
- Set the corresponding element of reverse to the current element of the original array.
- Convert reverse back to a string and then to an integer and return it.
public static int reverse(int num) { char[] arr = Integer.toString(num).toCharArray(); char[] reverse = new char[arr.length]; for (int i = 0; i < arr.length; i++) { reverse[i] = arr[arr.length - i - 1]; } return Integer.parseInt(new String(reverse));}
Dry Run Explanation:
Let’s say we want to reverse the number 12345. Here is how the method would work:
| Iteration | i | arr[arr.length – i – 1] | reverse[i] | New value of reverse[i] |
|---|---|---|---|---|
| 1 | 0 | 5 | 5 | |
| 2 | 1 | 4 | 4 | |
| 3 | 2 | 3 | 3 | |
| 4 | 3 | 2 | 2 | |
| 5 | 4 | 1 | 1 |
In the last line of code, the reverse char array is being converted back to a string using the new String(reverse) constructor. This creates a new string object that contains the same sequence of characters as the reverse array.
Then, the Integer.parseInt() method is being called on the string to parse it as an integer. This returns an int value that is the integer representation of the string. Finally, this int value is returned as the output of the reverse() method.
Method 4: Reverse a Number in Java Using Recursion
- If num is less than 10, return num.
- Otherwise, return the last digit of num concatenated with a recursive call to reverse the rest of num.
public static int reverse(int num) { if (num < 10) { return num; } return Integer.parseInt(num % 10 + "" + reverse(num / 10));}
Dry Run Explanation:
Let’s say we want to reverse the number 12345. Here is how the method would work:
| Iteration | num | num % 10 | reverse(num / 10) | New value of reverse |
|---|---|---|---|---|
| 1 | 12345 | 5 | 1234 | 5 |
| 2 | 1234 | 4 | 123 | 54 |
| 3 | 123 | 3 | 12 | 543 |
| 4 | 12 | 2 | 1 | 5432 |
| 5 | 1 | 1 | 54321 |
Time and Space Complexity Comparison
| Method | Time complexity | Space complexity |
|---|---|---|
| 1. Using a loop | O(n) | O(1) |
| 2. Using a string | O(n) | O(n) |
| 3. Using a character array | O(n) | O(n) |
| 4. Using recursion | O(log n) | O(log n) |
In terms of time complexity, the fourth method (using recursion) has the lowest time complexity, followed by the first method (using a loop). The second and third methods (using a string and a character array, respectively) have the same time complexity.
In terms of space complexity, the first method (using a loop) has the lowest space complexity, followed by the second and third methods (using a string and a character array, respectively). The fourth method (using recursion) has the highest space complexity.
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