Reverse Array in Java: Solution, Explanation & Practice

Reverse an array in place

Problem summary

Write a program that reverses an array. Your code should work for ANY array.

Starter code

public class Main {
    public static void main(String[] args) {
        int[] arr = {1, 2, 3, 4, 5}; // Test case 1
        
        // Reverse array in place
        // Print elements space-separated
    }
}

Expected output and test cases

Reverse [1,2,3,4,5]
```
5 4 3 2 1
```
Reverse [1,2,3,4]
```
4 3 2 1
```
Reverse [1]
```
1
```

Hints

Swap elements from both ends
Use two pointers: start and end
Stop when pointers meet in middle

Validated solution

Reveal Java solution

import java.util.Arrays;

public class Main {
    static void reverse(int[] values) {
        for (int left = 0, right = values.length - 1; left < right; left++, right--) {
            int temporary = values[left];
            values[left] = values[right];
            values[right] = temporary;
        }
    }

    public static void main(String[] args) {
        int[] values = {1, 2, 3, 4, 5};
        reverse(values);
        System.out.println(Arrays.toString(values).replace("[", "").replace("]", "").replace(",", ""));
    }
}

How to approach the problem

Two pointers identify the next unmatched values at the front and back. Swapping them and moving both inward preserves the already reversed outer portion without allocating another array.

Approach

Set left to 0 and right to length - 1.
Swap while left is strictly before right.
Move both pointers inward after each swap.

Time and space complexity

Time: O(n). Space: O(1).

Edge cases to test

Empty and single-element arrays need no swap.
The method mutates the supplied array; callers that need the original should copy first.

Common mistakes

Using left <= right and performing an unnecessary middle self-swap.
Updating one pointer but not the other.

Follow-up challenge

Reverse only a requested inclusive range and validate its bounds.

Related Arrays exercises

Practice all Arrays exercises · Run this idea in the Java compiler