Remove Duplicates in Java: Solution, Explanation & Practice

Remove duplicate elements from array

Problem summary

Write a program that removes duplicates from a sorted array and prints unique elements.

Starter code

public class Main {
    public static void main(String[] args) {
        int[] arr = {1, 1, 2, 2, 3, 4, 4, 5}; // Test case 1
        
        // Remove duplicates and print unique elements
        // Print space-separated
    }
}

Expected output and test cases

Unique from sorted array
```
1 2 3 4 5
```
From [1,1,1,2,2,3]
```
1 2 3
```
From [5,5,5,5]
```
5
```

Hints

Since sorted, duplicates are adjacent
Compare current with previous
Only print if different from last printed

Validated solution

Reveal Java solution

public class Main {
    static int uniqueLength(int[] sorted) {
        if (sorted.length == 0) return 0;
        int write = 1;
        for (int read = 1; read < sorted.length; read++) {
            if (sorted[read] != sorted[write - 1]) sorted[write++] = sorted[read];
        }
        return write;
    }

    public static void main(String[] args) {
        int[] values = {1, 1, 2, 2, 3, 4, 4, 5};
        int length = uniqueLength(values);
        for (int i = 0; i < length; i++) System.out.print(values[i] + (i + 1 == length ? "" : " "));
    }
}

How to approach the problem

Because the array is sorted, equal values are adjacent. Keep a write position for the compacted prefix and copy a value only when it differs from the last unique value written.

Approach

Return zero for an empty array.
Keep index zero as the first unique value.
Advance write only after copying a new value.

Time and space complexity

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

Edge cases to test

All duplicates compact to one value.
This contract requires sorted input; unsorted input needs a Set or sorting.

Common mistakes

Comparing to sorted[read - 1] after the array has been overwritten.
Printing the whole backing array instead of only the returned prefix.

Follow-up challenge

Preserve order while removing duplicates from an unsorted array with LinkedHashSet.

Related Arrays exercises

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