Migratory Birds using Counting Sort [O(N)].

migratory_birds_count_sort.cpp

// Problem link: https://www.hackerrank.com/challenges/migratory-birds/problem

#include <iostream>
#include <vector>
#include <algorithm>
#include <climits>
using namespace std;

#define ull uint64_t

void count_sort_rec(const vector<int> &ar, vector<int> &count, ull idx = 0) {
    if(idx == ar.size()) return;
    count[ar[idx]]++;
    count_sort_rec(ar, count, idx+1);
}

void count_check_rec(const vector<int> &count, ull &max_idx, int max_ele = INT_MIN, ull idx = 1) {
    if(idx == count.size()) return;
    
    if(max_ele < count[idx]) {
        max_ele = count[idx];
        max_idx = idx;
    }

    count_check_rec(count, max_idx, max_ele, idx+1);
}

void migratory_birds(const vector<int> &ar) {
    vector<int> count(6,0);
    count_sort_rec(ar, count);

    ull max_idx = 0;
    count_check_rec(count, max_idx);
    cout << max_idx;
}

int main() {
    int n;  cin >> n;
    
    vector<int> arr(n);
    for(int i = 0; i < n; ++i)
        cin >> arr[i];
    
    migratory_birds(arr);
    return 0;
}