Maximum sum

#include <iostream>
#include <vector>

using namespace std;

int main() {
    long long n;
    cin >> n;

    // Read the values
    vector<long long> v(n);
    for (long long i = 0; i < n; ++i)
        cin >> v[i];

    vector<long long> best(n, 0);
    vector<long long> acc(n, 0);

    // Find the first positive or the highest negative
    long long first = 0;
    long long highest_negative;
    while (v[first] < 0 and first < n)
    {
        if (first == 0)
            highest_negative = v[first];
        else if (v[first] > highest_negative)
            highest_negative = v[first];
        ++first;
    }

    // All negative numbers
    if (first == n)
        cout << highest_negative << endl;
    // Some positive number(s)
    else {
        best[first] = v[first];
        acc[first] = v[first];

        for (long long i = first+1; i < n; ++i) {
            if (v[i] < 0) { // Always better not to pick it
                best[i] = best[i - 1];
                acc[i] = best[i-1] + v[i];
            }
            else {
                if (best[i-1] > (acc[i-1] + v[i]) and best[i-1] > v[i]) {
                    best[i] = best[i - 1];
                    acc[i] = acc[i - 1] + v[i];
                }
                else if ((acc[i-1] + v[i]) > v[i]) {
                    best[i] = acc[i - 1] + v[i];
                    acc[i] = best[i];
                }
                else {
                    best[i] = v[i];
                    acc[i] = v[i];
                }
            }
        }

        cout << best[n - 1] << endl;
    }
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: WRONG ANSWER

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED