Finish LuoguP3805: manacher

Luogu/LuoguP3805.cpp

@@ -0,0 +1,46 @@
+// problem statement: https://www.luogu.com.cn/problem/P3805
+
+#include <bits/stdc++.h>
+
+constexpr size_t MAXN = 1.1e7;
+
+using namespace std;
+
+string str;
+
+size_t manacher(const string &src) {
+    string s;
+    s.push_back('#');
+    for (auto &&ch : src) {
+        s.push_back(ch);
+        s.push_back('#');
+    }
+    s.push_back('#');
+    size_t ans = 0;
+    vector<size_t> p(s.size());
+    p[0] = 1;
+    size_t id = 0, max_right = 0;
+    for (size_t i = 1; i < s.length(); i++) {
+        if (i < max_right) {
+            p[i] = min(p[2 * id - i], max_right - i + 1);
+        } else {
+            p[i] = 1;
+        }
+        while (i + p[i] < s.length() && i >= p[i] && s[i + p[i]] == s[i - p[i]]) {
+            p[i]++;
+        }
+        if (i + p[i] - 1 > max_right) {
+            max_right = i + p[i] - 1;
+            id = i;
+        }
+        ans = max(ans, p[i] - 1);
+    }
+    return ans;
+}
+
+int main() {
+    ios::sync_with_stdio(false);
+    cin >> str;
+    cout << manacher(str) << '\n';
+    return 0;
+}