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

    int solution(vector<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].

#include <unordered_map>

int solution(vector<int> &A) {
    // write your code in C++11 (g++ 4.8.2)
    unordered_map<int, int> um;
    
    for (size_t i = 0; i < A.size(); i++) {
        if (um.find(A[i]) != um.end()) {
            um[A[i]]++;
        } else {
            um[A[i]] = 1;
        }
        
        if (um[A[i]] > A.size() / 2) return i;
    }
    
    return -1;
}