题解，不喜勿喷

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main() {
int n;
cin >> n;
vector<int> a(n);
for (int i = 0; i < n; ++i) {
cin >> a[i];
}

vector<vector<long long>> dp(n, vector<long long>(n, 0));

for (int i = n - 1; i >= 0; --i) {
    for (int j = i; j < n; ++j) {
        if (i == j) {
            dp[i][j] = a[i];
        } else {
            long long left = a[i] - dp[i + 1][j];
            long long right = a[j] - dp[i][j - 1];
            dp[i][j] = max(left, right);
        }
    }
}

cout << dp[0][n - 1] << endl;

return 0;

}