[TIOJ 1276] 對稱之山 (manacher)

/* https://tioj.ck.tp.edu.tw/problems/1276 */

// #pragma GCC target("avx2")
#pragma GCC optimize("O3", "unroll-loops")

// #include <bits/extc++.h>
// using namespace __gnu_pbds;

#include <bits/stdc++.h>
using namespace std;

// #define int long long
#define double long double
// template <typename T>
// using pbds_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
using pii = pair<int, int>;
template<typename T>
using prior = priority_queue<T, vector<T>, greater<T>>;
template<typename T>
using Prior = priority_queue<T>;

#define X first
#define Y second
#define ALL(x) (x).begin(), (x).end()
#define eb emplace_back
#define pb push_back

#define fastIO() ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define RANDOM() random_device __rd; \
                 mt19937 __gen = mt19937(__rd()); \
                 uniform_int_distribution<int> __dis(0, 1); \
                 auto rnd = bind(__dis, __gen);

const int INF = 1E9;
const int mod = 1E9 + 7;
const int maxn = 2E7 + 5;

vector<int> palin(maxn, 0);

int32_t main() {
    // fastIO();

    int n, sz;
    scanf("%d", &n), sz = 2*n + 1;

    vector<int> v(sz, INF);
    for (int i = 1; i < sz; i += 2) scanf("%d", &v[i]);

    int maxL = 0, maxR = -1;
    for (int i = 0; i < sz; ++i) {
        if (i > maxR) {
            for (int len = 0; i-len >= 0 and i+len < sz; ++len) {
                if (v[i-len] == v[i+len]) ++palin[i];
                else break;
            }
            maxL = i - palin[i] + 1;
            maxR = i + palin[i] - 1;
        }
        else {
            int mir = maxL + maxR - i;
            palin[i] = min(maxR - i, palin[mir]);

            if (palin[i] == maxR - i) {
                for (int len = palin[i]; i-len >= 0 and i+len < sz; ++len) {
                    if (v[i-len] == v[i+len]) ++palin[i];
                    else break;
                }
                maxL = i - palin[i] + 1;
                maxR = i + palin[i] - 1;
            }
        }
    }

    int ans = 0;
    for (int i = 0; i < sz; ++i) ans = max(ans, palin[i] - 1);

    printf("%d\n", ans-1);

    return 0;
}