Use objective-c write code to return the first repeated integer from a given array, with O(n) time

Question

The Dude · Accepted Answer

Use a Hash Table to insert each element (in our case number) - search and insert  of each of the N element is O(1)

Search and Insert all N elements is O(N)

Whenever a given elements already exist at the Hash Table output this element

O(N) · Answer

private static void findDuplicate() {
		
		int arrayInt[] = { 2, 4, 56, 2, 4, 7, 8 };
		HashMap map = new HashMap();
		try {
			for (int i = 0; i < arrayInt.length; i++) {
				if (!map.containsKey(arrayInt[i])) {
					map.put(arrayInt[i], arrayInt[i]);

				}
				else {
					System.out.println("duplicate item:" + arrayInt[i] + " at index: " + i);
				}
			}
		}
		catch (Exception e) {
			e.printStackTrace();
		}
	}

Matteo · Answer

A NSSet is enough.

Anonymous · Answer

if the array is sorted, you could XOR consecutive integers and return element whose XOR result is 0

Anonymous · Answer

Is there any solution better than hashtable?

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