https://www.hackerrank.com/challenges/angry-children

Among N ints, pick K with min 'unfairness' (max of k - min of k). Here's the strategy: larger numbers should be picked together with other larger numbers, and only by this can we make sure a smaller unfairness - that means: sorting. Then we can go through each sorted K section: data[i + k - 1] - data[i]:

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;

int main() 
{
    int N, K, unfairness;
    cin >> N >> K;    
    vector<int> candies(N);
    for (int i=0; i<N; i++)
        cin >> candies[i];
    
    std::sort(candies.begin(), candies.end());
    unfairness = std::numeric_limits<int>::max();

    for(int i = 0; i <= N - K ; i ++)
    {
        unfairness = std::min(unfairness, candies[i + K - 1] - candies[i]);
    }

    cout << unfairness << "\n";
    return 0;
}