21C - Stripe 2 - CodeForces Solution

// Mtaylor
#include <bits/stdc++.h>
using namespace std;

#define pb push_back
#define fi first
#define se second
#define mp make_pair
#define endl '\n';
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
void dbg_out() { cerr << endl; }
template <typename Head, typename... Tail>
void dbg_out(Head H, Tail... T) {
    cerr << ' ' << H;
    dbg_out(T...);
}
#define dbg(...) cerr << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)
typedef long long ll;
const int N = 3e5 + 5;

ll sum[N];
ll a[N];
int n;
ll s;
map<ll, int> freq;
void test_case() {
    cin >> n;
    for (int i = 1; i <= n; i++) {
        cin >> a[i];
        sum[i] = a[i] + sum[i - 1];
        freq[sum[i]]++;
        s += a[i];
    }
    ll cs = 0;
    freq[sum[n]]--;
    ll ans = 0;
    for (int i = n; i >= 2; i--) {
        freq[sum[i - 1]]--;
        cs += a[i];
        if (cs * 3 == s) {
            ans += freq[cs];
        }
    }
    cout << ans << endl;
}

int main() {
    // freopen("input.txt", "r", stdin);
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    int T;
    // cin >> T;
    T = 1;
    while (T--) {
        test_case();
    }
    return 0;
}