[codility] Dominator

An array A consisting of N integers is given. The dominator of array A is the value that occurs in more than half of the elements of A.

For example, consider array A such that

A[0] = 3 A[1] = 4 A[2] = 3 A[3] = 2 A[4] = 3 A[5] = -1 A[6] = 3 A[7] = 3

The dominator of A is 3 because it occurs in 5 out of 8 elements of A (namely in those with indices 0, 2, 4, 6 and 7) and 5 is more than a half of 8.

Write a function

class Solution { public int solution(int[] A); }

that, given an array A consisting of N integers, returns index of any element of array A in which the dominator of A occurs. The function should return −1 if array A does not have a dominator.

For example, given array A such that

A[0] = 3 A[1] = 4 A[2] = 3 A[3] = 2 A[4] = 3 A[5] = -1 A[6] = 3 A[7] = 3

the function may return 0, 2, 4, 6 or 7, as explained above.

Write an efficient algorithm for the following assumptions:

• N is an integer within the range [0..100,000];
• each element of array A is an integer within the range [−2,147,483,648..2,147,483,647].

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;



public static int solution(int[] A) {

        Map<Integer, Integer> map = new HashMap<>();

        for (int a : A) {
            map.put(a, map.getOrDefault(a, 0) + 1);
        }

        List<Map.Entry<Integer, Integer>> list = new ArrayList<>(map.entrySet());
        list.sort(Map.Entry.<Integer, Integer>comparingByValue().reversed());

        int ret = -1;

        if(list.size() < 1){
            return ret;
        }

        if (map.get(list.get(0).getKey()) > A.length / 2) {
            int key = list.get(0).getKey();
            int[] R = new int[map.get(list.get(0).getKey())];
            int j = 0;
            for (int i = 0; i < A.length; i++) {
                if (A[i] == key) {
                    R[j] = i;
                    j++;

                }
            }
            ret = R[0];
        }
        return ret;
}