Solution based on this answer
#include <algorithm>
#include <iterator>
#include <iostream>
#include <numeric>
#include <ostream>
#include <utility>
#include <vector>
// __________________________________________________
template<typename RandomAccessIterator> typename std::iterator_traits<RandomAccessIterator>::value_type
max_subarr_k(RandomAccessIterator first,RandomAccessIterator last,int k)
{
using namespace std;
typedef typename iterator_traits<RandomAccessIterator>::value_type value_type;
if(distance(first,last) < k)
return value_type(0);
RandomAccessIterator tail=first;
first+=k;
value_type window=accumulate(tail,first,value_type(0));
value_type max_sum=window, current_sum=window;
while(first!=last)
{
window += (*first)-(*tail) ;
current_sum = max( current_sum+(*first), window );
max_sum = max(max_sum,current_sum);
++first;
++tail;
}
return max_sum;
}
// __________________________________________________
template<typename E,int N>
E *end(E (&arr)[N])
{
return arr+N;
}
int main()
{
using namespace std;
int arr[]={1,2,4,-5,-4,-3,2,1,5,6,-20,1,1,1,1,1};
cout << max_subarr_k(arr,end(arr),4) << endl;
cout << max_subarr_k(arr,end(arr),5) << endl;
}
Output is:
14
11