1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
#include<bits/stdc++.h>
using namespace std;

struct custom_hash {
    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return x + FIXED_RANDOM;
    }
};

void solve() {
    int n; cin >> n;
    vector<int> a(n + 1, 0);
    for(int i = 1; i <= n; i++) cin >> a[i];
    vector<int> pref(n + 1, 0), suf(n + 1, 0);
    for(int i = 1; i <= n; i++) pref[i] = pref[i - 1] + a[i];
    for(int i = n-1; i >= 0; i--) suf[i] = suf[i + 1] + a[i + 1];
    vector<int> presuf;
    for(int a = 0; a < n; a++) {
        for(int b = n - a - 1; b >= 0; b--) {
            presuf.push_back(pref[a] + suf[n - b]);
            }
    }
    //cout << "\n";
    unordered_map<int, int, custom_hash> rhs;
    long long ans = 0;
    for(int i = n - 1; i >= 0; i--) {
        // Dodajemy 2 polowki do zbioru
        for(auto x : presuf) {
            rhs[x + suf[i + 1]]++;
        }
        for(auto x : presuf) {
            if(rhs.count(3 * pref[n] - pref[i] - x) != 0) {
                //cout << "lval: " << x << " lpref: " << pref[i] << " rval + rsuf: " << 3 * pref[n] - pref[i] - x << "\n";
                ans += rhs[3 * pref[n] - pref[i] - x];
            }
        }
    }
    //cout << ans << "\n";
    unordered_map<int, int, custom_hash> powt;
    //for(auto x : presuf) cout << x << " ";
    //cout << "\n";
    for(auto x : presuf) powt[x]++;
    for(auto x : presuf) {
        if(powt.count(3 * pref[n] - 2 * x) != 0) {
            ans -= 3 * powt[3 * pref[n] - 2 * x];
        }
    }
    //cout << ans << "\n";
    ans += 2 * powt[pref[n]];
    //cout << ans << "\n";
    cout << ans / 6 << "\n";
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    solve();
    return 0;
}