#1821. 花海漫步

https://sjzezoj.com/submission/67054

S2OJ/1821/1821.cpp

@@ -0,0 +1,71 @@
+#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;
+}