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up vote 6 down vote favorite

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]);  
    }
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1  
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? –  Tony R Jun 13 '12 at 15:26
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2 Answers

up vote 7 down vote accepted

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()]);
}
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It doesn't work if number exists in the array 3, 5, and etc (odd number) times. –  Andrew Jun 1 '12 at 4:25
    
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). –  Donald.McLean Jun 1 '12 at 4:50
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up vote 5 down vote

More compact version :)

 public static Integer[] outputSinglePair(Integer[] numbers){
        if(numbers == null)
            throw new IllegalArgumentException();

        HashSet<Integer> result = new HashSet<Integer>();
        for (int next: numbers){
            if (!result.add(next))
                result.remove(next);
        }
        return result.toArray(new Integer[result.size()]);
    }
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