P4555 [国家集训队]最长双回文串

https://www.luogu.com.cn/record/77999110

Luogu/P4555/P4555.cpp

@@ -0,0 +1,57 @@
+#include <iostream>
+#include <string>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 1e5 + 5;
+
+int mid, r, p[N << 1], a[N << 1], b[N << 1], ans;
+std::string s1, s2;
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> s1;
+
+    s2.push_back('^');
+
+    for (const char &c : s1) {
+        s2.push_back('#');
+        s2.push_back(c);
+    }
+
+    s2 += "#$";
+
+    for (int i = 1; i < s2.size(); i++) {
+        p[i] = i < r ? std::min(p[mid * 2 - i], r - i) : 1;
+        while (s2[i - p[i]] == s2[i + p[i]]) p[i]++;
+        if (i + p[i] > r) {
+            r = i + p[i];
+            mid = i;
+        }
+
+        a[i + p[i] - 1] = std::max(a[i + p[i] - 1], p[i] - 1);
+        b[i - p[i] + 1] = std::max(b[i - p[i] + 1], p[i] - 1);
+    }
+
+    for (int i = 3; i <= s2.size() - 4; i += 2) {
+        b[i] = std::max(b[i], b[i - 2] - 2);
+    }
+
+    for (int i = s2.size() - 4; i >= 3; i -= 2) {
+        a[i] = std::max(a[i], a[i + 2] - 2);
+    }
+
+    for (int i = 3; i <= s2.size() - 4; i += 2) {
+        if (a[i] && b[i]) {
+            ans = std::max(ans, a[i] + b[i]);
+        }
+    }
+
+    cout << ans << endl;
+
+    return 0;
+}