if you cannot solve this problem I give you my solution. The solution is simple:)
#include <iostream>
using namespace std;
int main()
{
 int a;
 int n;
cin>>n;
int sum=0;
int max1=0;
 while (n--){
  cin>>a;
   sum+=a;
  if (sum<0)
   sum=0;
  max1=max(sum,max1);

  }
cout<<max1<<endl;

}

i can't solve this prblm with O(N)!! thank you!