题解 | 能量项链

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

typedef long long ll;
const int N = 220;
const int Mod = 1e9 + 7;
ll a[N];
int n;
ll dp[N][N];

int main() {
    cin >> n;
    for (int i = 1; i <= n; i++) {
        cin >> a[i];
        a[i + n] = a[i];
    }

    for (int len = 3; len <= n + 1; len++) {
        for (int i = 1, j; (j = i + len - 1) <= 2 * n; i++) {
            for (int k = i + 1; k < j; k++) {
                dp[i][j] = max(dp[i][j], dp[i][k] + dp[k][j] + a[i] * a[k] * a[j]);
            }
        }
    }

    ll ans = 0;
    for (int i = 1; i <= n; i++) {
        ans = max(ans, dp[i][n + i]);
    }

    cout << ans % Mod;

    return 0;
}