missingNumbers

// https://www.hackerrank.com/challenges/missing-numbers/problem

// this solution posted as comment on, to explain how O(n) solution is designed: 
//    = https://www.youtube.com/watch?v=N6r9Dh68Y3o

vector<int> missingNumbers(vector<int> arr, vector<int> brr) {
    auto const base = brr.front();
    int freqs[100] {0};
    int actual_base = base;
    for(auto const item : brr) {
        auto const index = (100 - base + item) % 100;
        ++freqs[index];
        actual_base = std::min(actual_base, item);
    }
    for(auto const item : arr) {
        auto const index = (100 - base + item) % 100;
        --freqs[index];
    }
    vector<int> result;
    auto index = (100 - base + actual_base) % 100;
    for(auto i = 0; i < 100; ++i) {
        if(freqs[index]) {
            result.push_back(actual_base);
        }
        index = (index + 1) % 100;
        actual_base++;
    }
    return result;
}