Dominator

// you can use includes, for example:
// #include <algorithm>

// you can write to stdout for debugging purposes, e.g.
// cout << "this is a debug message" << endl;

int solution(vector<int> &A) {
    // write your code in C++14 (g++ 6.2.0)
    auto n = A.size();
    int size = 0;
    
    int value = 0;
    for (size_t k = 0; k < A.size(); ++k) {
        if (size == 0) {
            size++;
            value = A[k];
        }
        else {
            if (value != A[k]) {
                size--;
            }
            else {
                size++;
            }
        }
    }
    
    int candidate = -1;
    if (size > 0) {
        candidate = value;
    }
    
    int leader = -1;
    int count = 0;
    int index = 0;
    
    for (size_t k = 0; k < A.size(); ++k) {
        if (A[k] == candidate) {
            count++;
            index = k;
        }
    }
    if (count > (int)(n / 2)) {
        leader = candidate;
    }
    return leader > 0 ? index : -1;
}