fix manacher

Author: hitonanode

string/manacher.hpp

@@ -3,7 +3,6 @@
 #include <utility>
 #include <vector>
 
-// CUT begin
 // Manacher's Algorithm: radius of palindromes
 // Input: std::string or std::vector<T> of length N
 // Output: std::vector<int> of size N
@@ -30,8 +29,13 @@ std::vector<int> manacher(const std::string &S) {
     return manacher(v);
 }
 
+// Find maximal palindrome length for each center
+// input: array of length N
+// output: array of length N * 2 - 1
 template <typename T>
 std::vector<std::pair<int, int>> enumerate_palindromes(const std::vector<T> &vec) {
+    if (vec.empty()) return {};
+
     std::vector<T> v;
     const int N = vec.size();
     for (int i = 0; i < N - 1; i++) {
@@ -52,7 +56,6 @@ std::vector<std::pair<int, int>> enumerate_palindromes(const std::vector<T> &vec
     }
     return ret;
 }
-
 std::vector<std::pair<int, int>> enumerate_palindromes(const std::string &S) {
     std::vector<char> v(S.size());
     for (int i = 0; i < int(S.size()); i++) v[i] = S[i];

string/manacher.md

@@ -16,6 +16,11 @@ for (auto [l, r] : palindromes) {
 }
 ```
 
+## 問題例
+
+- [Enumerate Palindromes](https://judge.yosupo.jp/problem/enumerate_palindromes)
+- [No.3392 Count 23578 Sequence - yukicoder](https://yukicoder.me/problems/no/3392)
+
 ## 文献
 
 - [1] G. Manacher, "A new linear-time "on-line" algorithm for finding the smallest initial palindrome of a string", Journal of the ACM, 22 (3), 346-351, 1975.

string/test/manacher.yuki3392.test.cpp

@@ -0,0 +1,22 @@
+#define PROBLEM "https://yukicoder.me/problems/no/3392"
+#include "../manacher.hpp"
+
+#include <iostream>
+using namespace std;
+
+int main() {
+    cin.tie(nullptr), ios::sync_with_stdio(false);
+
+    int N;
+    cin >> N;
+    vector<int> A(N);
+    for (auto &a : A) cin >> a;
+
+    vector<int> D(N - 1);
+    for (int i = 0; i < N - 1; ++i) D.at(i) = A.at(i + 1) - A.at(i);
+
+    long long ret = N;
+    for (auto [l, r] : enumerate_palindromes(D)) ret += (r - l + 1) / 2;
+
+    cout << ret << '\n';
+}