题目链接

利用回文树求前缀后缀的回文子串数，枚举计算答案即可。

int main() {
    #ifdef LOCAL
        INPUT;
        OUTPUT;
    #endif

   // scanf("%d", &n);
    scanf("%s", S + 1);
n=strlen(S+1);
    LL cnt = 0;
    memset(R, 0, sizeof R);
    for (int i = 1, l = 0, r = 0; i <= n; ++i) {
        if (i <= r) R[i] = min(r - i + 1, R[l + r - i]);
        while (1 <= i - R[i] && i + R[i] <= n && S[i - R[i]] == S[i + R[i]]) R[i]++;
        if (i + R[i] - 1 > r) l = i - R[i] + 1, r = i + R[i] - 1;
        cnt += R[i];
        Start[i - R[i] + 1]++; End[i]++;
        Start[i + 1]--; End[i + R[i]]--;
        //printf("%d ", R[i]);
    }
    //puts("");

    memset(R, 0, sizeof R);
    for (int i = 1, l = 0, r = 0; i <= n - 1; ++i) {
        if (i < r) R[i] = min(r - i, R[l + r - i - 1]);
        while (1 <= i - R[i] && i + R[i] < n && S[i - R[i]] == S[i + R[i] + 1]) R[i]++;
        if (i + R[i] > r) l = i - R[i] + 1, r = i + R[i];
        cnt += R[i];
        Start[i - R[i] + 1]++; End[i + 1]++;
        Start[i + 1]--; End[i + R[i] + 1]--;
        //printf("%d ", R[i]);
    }
    //puts("");

    LL ans =0;
    for (int i = 1; i <= n; ++i) Start[i] = (Start[i] + Start[i - 1]) % M;
    for (int i = 1; i <= n; ++i) End[i] = (End[i] + End[i - 1]) % M;
    for (int i = 1; i <= n; ++i) End[i] = (End[i] + End[i - 1]) % M;

    for (int i = 1; i <= n; ++i) ans = (ans + Start[i] * End[i - 1] % M + M) % M;
    cout << ans;
    return 0;
}