Finding Maximal-Sum Positive Integer Subarray

You are given an integer array consisting of N positive integers. You have to pick a subarray of length K. Subarray means a contiguous part of an array. You need to find the sum of the subarray with the maximal sum among all such subarrays.

Input should be given as follows. On the first line, N and K: the size of the array, the size of the subarray to choose. On the next line: N integers, Ai.
The output should be the answer, the sum of the subarray that has the maximal sum.

Constraints
1 ≤ K ≤ N ≤ 1000 and 1 ≤ Ai ≤ 1000

I solved it like this but 2 tests give errors. What have I overlooked?

#include<iostream>
using namespace std;
int main(){
    ios_base::sync_with_stdio(NULL);
    cin.tie(NULL);
    int n,k;
    cin>>n>>k;
    int a,s[n],sum=0,ans=0,ans1;
    for(int i=0; i<n; i++){
        cin>>a;
        sum+=a; 
        s[i]=sum;
    }
    if(k==n){
      cout<<sum<<endl;
      return 0;
    }
    for(int i=n-1; i>=0; i--){
       if(i-k>=0){
         ans1=s[i]-s[i-k];
       }
         ans=max(ans,ans1);
    }
    cout<<ans<<endl;
    return 0;}

https://algoleague.com/problem/max-subarray-2/detail

Without an example that reproduces the issue, it is extremely difficult to debug an algorithm. You probably feel stuck because I presume the only feedback you got was that your submission failed a certain number of tests. As you might guess, that is also frustrating for someone trying to answer your question.

This is not a criticism of you trying to get help with your question, but a general commentary on the types of questions that appear from some programming competition sites which appear not to give sufficient feedback to encourage good debugging techniques.

Given that is the situation, here is an idea. I read your code and I cannot find any issues. I also tested it with some sample data I generated and it passed all the tests. Below, you will find a different implementation that uses a sliding window. Maybe by comparing the two strategies, you can try to figure out if there are actually any differences.

int main(int argc, const char *argv[]) {
    int count, k;
    cin >> count >> k;

    int sum{};
    int arr[k];
    for (auto i = 0; i < k; ++i) {
        cin >> arr[i];
        sum += arr[i];
    }

    int max_sum{sum};
    for (auto i = k; i < count; ++i) {
        auto idx = i % k;
        sum -= arr[idx];
        cin >> arr[idx];
        sum += arr[idx];
        max_sum = std::max(max_sum, sum);
    }

    cout << max_sum << endl;
    return 0;
}