Find the most popular element in int[] array

Try this answer. First, the data:

int[] a = {1,2,3,4,5,6,7,7,7,7};

Here, we build a map counting the number of times each number appears:

Map<Integer, Integer> map = new HashMap<Integer, Integer>();
for (int i : a) {
    Integer count = map.get(i);
    map.put(i, count != null ? count+1 : 1);
}

Now, we find the number with the maximum frequency and return it:

Integer popular = Collections.max(map.entrySet(),
    new Comparator<Map.Entry<Integer, Integer>>() {
    @Override
    public int compare(Entry<Integer, Integer> o1, Entry<Integer, Integer> o2) {
        return o1.getValue().compareTo(o2.getValue());
    }
}).getKey();

As you can see, the most popular number is seven:

System.out.println(popular);
> 7

EDIT

Here’s my answer without using maps, lists, etc. and using only arrays; although I’m sorting the array in-place. It’s O(n log n) complexity, better than the O(n^2) accepted solution.

public int findPopular(int[] a) {

    if (a == null || a.length == 0)
        return 0;

    Arrays.sort(a);

    int previous = a[0];
    int popular = a[0];
    int count = 1;
    int maxCount = 1;

    for (int i = 1; i < a.length; i++) {
        if (a[i] == previous)
            count++;
        else {
            if (count > maxCount) {
                popular = a[i-1];
                maxCount = count;
            }
            previous = a[i];
            count = 1;
        }
    }

    return count > maxCount ? a[a.length-1] : popular;

}