From 7a4c540059ddf4d9ecf999ee21e32c6c060c88ab Mon Sep 17 00:00:00 2001
From: Baoshuo <i@baoshuo.ren>
Date: Sun, 26 Jun 2022 21:48:42 +0800
Subject: [PATCH] =?UTF-8?q?3188.=20manacher=E7=AE=97=E6=B3=95?=
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https://www.acwing.com/problem/content/description/3190/
---
 AcWing/3188/3188.cpp | 52 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)
 create mode 100644 AcWing/3188/3188.cpp

diff --git a/AcWing/3188/3188.cpp b/AcWing/3188/3188.cpp
new file mode 100644
index 00000000..0afeeeba
--- /dev/null
+++ b/AcWing/3188/3188.cpp
@@ -0,0 +1,52 @@
+#include <iostream>
+#include <string>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 1e7 + 5;
+
+int p[N << 1], mid, r, ans;
+std::string s1, s2;
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> s1;
+
+    // 使字符串的长度变为 (2n + 1)，方便处理
+    s2.push_back('^');
+    for (const char &c : s1) {
+        s2.push_back('#');
+        s2.push_back(c);
+    }
+    s2 += "#$";
+
+    for (int i = 1; i < s2.size(); i++) {
+        // r 代表以 mid 为中心的最长回文子串的右边界
+        p[i] = i < r
+                 // mid * 2 - i 为 i 关于 mid 的对称点
+                 ? std::min(p[mid * 2 - i], r - i)
+                 // 超过边界就不是回文串了
+                 : 1;
+
+        // 扩展回文串长度
+        while (s2[i - p[i]] == s2[i + p[i]]) p[i]++;
+
+        // 扩展右边界
+        if (r < i + p[i]) {
+            r = i + p[i];
+            mid = i;
+        }
+    }
+
+    for (int i = 0; i < s2.size(); i++) {
+        ans = std::max(ans, p[i] - 1);  // p[i] - 1 即为以 i 为中心的最长的回文子串长度
+    }
+
+    cout << ans << endl;
+
+    return 0;
+}