Remove duplicates in array

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Time: O(n)

Space: O(1)

Problem Overview

Remove duplicates from unsorted array.

Intuition

Remove duplicates from unsorted array. HashSet approach.

Algorithm

1Iterate array. Add to set if not seen. Build result from elements first seen.

Common Pitfalls

• For sorted: two-pointer. For unsorted: hashset maintaining insertion order. O(n).

Remove duplicates in array.java

Java

// Approach: Two-pointer technique. Move unique elements forward in-place.
// Time: O(n) Space: O(1)
import java.util.*;

class Solution {
    ArrayList<Integer> removeDuplicate(int arr[]) {
        HashSet<Integer> seen = new HashSet<>();
        ArrayList<Integer> result = new ArrayList<>();
        for (int num : arr) {
            // If the element is not in the set, add it to both the set and the result list
            if (!seen.contains(num)) {
                seen.add(num);
                result.add(num);
            }
        }

        return result;
    }
}

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