Find all single elements in an array

Question

I am interested in printing all numbers that do not have a match in an array.
Example: 1,3,4,5,3,4,5,5,6 result 1,5,6
Please review my solution bellow. What would be a much better way for this? Any input on how to improve this is welcome

public static Integer[] outputSinglePair(Integer[] numbers){ 
 if(numbers == null) 
 throw new IllegalArgumentException(); 
 Arrays.sort(numbers); 
 ArrayList<Integer> result = new ArrayList<Integer>(); 
 for(int i = 0; i < numbers.length - 1; ){ 
 if(numbers[i] != numbers[i + 1]){ 
 result.add(numbers[i]); 
 i++; 
 } 
 else 
 i+=2; 
 if(i == numbers.length - 1)result.add(numbers[i]);//we hit last element of array which is unpaired
 } 
 return result.toArray(new Integer[0]); 
 } 

• Tony R: Are you sure that you have phrased this question correctly? "am interested in printing all numbers that do not have a match in an array." Based on your input, 1 appears once, 3 appears twice, 4 appears twice, 5 appears thrice, and 6 appears once. I would expect that only numbers 1 and 6 appear in the result set yet you have included 5. Can you please clarify?

Answer 1

I think this is the sort of thing for which HashSet is perfect. Your implementation would look something like I have shown below. The advantage is that you don't need a sort so your running time is strictly linear.

(Updated to fix syntax errors)

public static Integer[] outputSinglePair(Integer[] numbers){ 
 if(numbers == null)
 throw new IllegalArgumentException();
 HashSet<Integer> result = new HashSet<Integer>();
 for (int next: numbers) {
 if (result.contains(next)) {
 result.remove(next);
 }
 else {
 result.add(next);
 }
 }
 return result.toArray(new Integer[result.size()]);
} 

• anstarovoyt: It doesn't work if number exists in the array 3, 5, and etc (odd number) times.
• Donald.McLean: Actually, it does work for odd number times. When you have three of something - first one goes into result, second one takes it out, third puts it back in. I tested it just to make sure (and fix a couple of syntax errors).