mirror of
https://git.sb/baoshuo/OI-codes.git
synced 2024-12-25 06:11:59 +00:00
72 lines
1.5 KiB
C++
72 lines
1.5 KiB
C++
#include <iostream>
|
|
#include <numeric>
|
|
#include <string>
|
|
#include <vector>
|
|
|
|
using std::cin;
|
|
using std::cout;
|
|
const char endl = '\n';
|
|
|
|
const int N = 2e6 + 5;
|
|
const int mod = 1e9 + 7;
|
|
|
|
std::vector<int> manacher(const std::string &_s) {
|
|
int n = _s.size();
|
|
std::string s(n * 2 + 3, '#');
|
|
|
|
for (int i = 0; i < n; i++) {
|
|
s[i * 2 + 2] = _s[i];
|
|
}
|
|
|
|
s[0] = '^';
|
|
s[n * 2 + 2] = '$';
|
|
|
|
std::vector<int> p(s.size(), 1);
|
|
|
|
for (int i = 1, mid = 0, r = 0; i <= n * 2 + 1; i++) {
|
|
p[i] = i < r ? std::min(p[mid * 2 - i], r - i) : 1;
|
|
|
|
while (s[i - p[i]] == s[i + p[i]]) p[i]++;
|
|
|
|
if (i + p[i] - 1 > r) {
|
|
r = i + p[i] - 1;
|
|
mid = i;
|
|
}
|
|
}
|
|
|
|
return p;
|
|
}
|
|
|
|
int main() {
|
|
std::ios::sync_with_stdio(false);
|
|
cin.tie(nullptr);
|
|
|
|
int n;
|
|
long long sum = 0, ans = 0;
|
|
std::string s;
|
|
|
|
cin >> n >> s;
|
|
|
|
auto p = manacher(s);
|
|
std::vector<long long> pre(p.size()), suf(p.size());
|
|
|
|
for (int i = 2; i < p.size() - 2; i++) {
|
|
pre[i - p[i] + 1]++, pre[i + 1]--;
|
|
suf[i]++, suf[i + p[i]]--;
|
|
ans = (ans + p[i] / 2) % mod;
|
|
}
|
|
|
|
ans = ans * (ans - 1) / 2;
|
|
std::partial_sum(pre.begin(), pre.end(), pre.begin());
|
|
std::partial_sum(suf.begin(), suf.end(), suf.begin());
|
|
|
|
for (int i = 2; i < p.size() - 2; i += 2) {
|
|
ans = ((ans - sum * pre[i] % mod) % mod + mod) % mod;
|
|
sum = (sum + suf[i]) % mod;
|
|
}
|
|
|
|
cout << ans << endl;
|
|
|
|
return 0;
|
|
}
|