This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
using namespace std;
#define fast ios::sync_with_stdio(0);cin.tie(0);
typedef long long ll;
#define f first
#define s second
#define MOD 998244353
#define LOGN 21
#define MAXN 50005
const ll P = 31;
vector<vector<int>> graph;
string s;
ll marked[MAXN], sz[MAXN], powP[MAXN];
bool ch = false;
int get_sz(int node, int parent) {
sz[node] = 1;
for (auto u : graph[node]) {
if (u == parent || marked[u])
continue;
sz[node] += get_sz(u, node);
}
return sz[node];
}
int find_centro(int node, int parent, int n) {
for (auto u : graph[node]) {
if (u != parent && !marked[u] && sz[u] * 2 >= n)
return find_centro(u, node, n);
}
return node;
}
int ref_val;
set<ll> hashes[MAXN];
void eval(int node, int parent, ll normal_hash, ll rev_hash, ll dist) {
if (ch || dist > ref_val)
return ;
normal_hash = (normal_hash * P + (s[node] - 'a' + 1)) % MOD;
rev_hash = (rev_hash + powP[dist - 1] * (s[node] - 'a' + 1)) % MOD;
if (rev_hash == normal_hash && dist == ref_val) {
ch = true;
return ;
}
if (ref_val == dist)
return ;
ll hash_req = (rev_hash * powP[ref_val - dist] - normal_hash + MOD) % MOD;
if (hashes[ref_val - dist].count(hash_req))
ch = true;
for (auto u : graph[node]) {
if (u == parent || marked[u])
continue;
eval(u, node, normal_hash, rev_hash, dist + 1);
}
}
void add(int node, int parent, ll normal_hash, ll rev_hash, ll dist) {
if (ch || dist > ref_val)
return ;
normal_hash = (normal_hash * P + (s[node] - 'a' + 1)) % MOD;
rev_hash = (rev_hash + powP[dist - 1] * (s[node] - 'a' + 1)) % MOD;
if (rev_hash == normal_hash && dist == ref_val) {
ch = true;
return ;
}
if (ref_val == dist)
return ;
ll hash_this = (rev_hash * powP[ref_val - dist] - normal_hash + MOD) % MOD;
hashes[dist].insert(hash_this);
for (auto u : graph[node]) {
if (u == parent || marked[u])
continue;
add(u, node, normal_hash, rev_hash, dist + 1);
}
}
void decompose(int node) {
int n = get_sz(node, node);
int centro = find_centro(node, node, n);
marked[centro] = true;
for (int i = 0; i <= sz[node]; i++)
hashes[i].clear();
for (auto u : graph[centro]) {
if (!marked[u]) {
eval(u, centro, s[centro] - 'a' + 1, s[centro] - 'a' + 1, 2);
add(u, centro, 0, 0, 1);
}
}
for (auto u : graph[centro]) {
if (!marked[u])
decompose(u);
}
}
bool check(int mid) {
ch = 0, ref_val = mid;
for (int i = 0; i < MAXN; i++)
marked[i] = false;
decompose(1);
return ch;
}
int main() {
fast
powP[0] = 1;
for (int i = 1; i < MAXN; i++)
powP[i] = (powP[i-1] * P) % MOD;
int N, a, b;
cin >> N >> s;
s = "#" + s;
graph = vector<vector<int>>(N+1, vector<int>());
for (int i = 1; i < N; i++) {
cin >> a >> b;
graph[a].push_back(b);
graph[b].push_back(a);
}
int L = 0;
int R = (N - 1) / 2;
int ans = -1;
while (R >= L) {
int mid = L + (R - L) / 2;
if (check(2 * mid + 1)) {
ans = 2 * mid + 1;
L = mid + 1;
} else
R = mid - 1;
}
int ult_ans = ans;
L = 1;
R = N / 2;
ans = -1;
while (R >= L) {
int mid = L + (R - L) / 2;
if (check(2 * mid)) {
ans = 2 * mid;
L = mid + 1;
} else
R = mid - 1;
}
cout << max(1, max(ans, ult_ans)) << "\n";
}
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