字串

struct Node {
    vector<int> c;
    Node(): c(26, -1) {}
    int &operator()(char nxt) {
        return c[nxt - 'a'];
    }
};
vector<Node> trie;
int add(string &str, int now, int id) {
    //  cout<<now<<"now"<<id<<"id\n";
    if (id == -1) {
        id = tmp++;
    }

    if (now == str.size())
        return id;

    trie[id](str[now]) = add(str, now + 1, trie[id](str[now]));
    return id;
}
int search(string &str, int now, int id) {
    if (now == str.size())
        return now;

    char c = str[now];

    if (trie[id](c) != -1) {
        return search(str,now+1,trie[id](c));
    }
    return now+1;
}

//Author: Woody
#include <bits/stdc++.h>
#define int long long
#define mp make_pair
#define eb emplace_back
#define rep(n) for(int i=0;i<n;i++)
#define rep2(n) for(int j=0;j<n;j++)
#define F first
#define S second
#define all(v) v.begin(),v.end()
#define SZ(x) (int)(x.size())
#define lowbit(x) (x&-x)
#define SETIO(s) ifstream cin(s+".in");ofstream cout(s+".out");
#define quick ios::sync_with_stdio(0);cin.tie(0);
using namespace std;
typedef pair<int, int> pii;
template <class t1, class t2>
inline const pair<t1, t2> operator + (const pair<t1, t2> &p1, const pair<t1, t2> &p2) {
    return pair<t1, t2>(p1.F + p2.F, p1.S + p2.S);
}
template <class t1, class t2>
inline const pair<t1, t2> operator - (const pair<t1, t2> &p1, const pair<t1, t2> &p2) {
    return pair<t1, t2>(p1.F - p2.F, p1.S - p2.S);
}
const int INF = 1e18;
const int N = 1e5 + 7;
const int D = 31;
string s[N];
string bin(int num) {
    string S;

    while (num > 0) {
        if (num & 1)
            S += "1";
        else
            S += "0";

        num >>= 1;
    }

    S += string(D - S.size(), '0');
    reverse(all(S));
    return S;
}
int tmp = 1;
struct Node {
    vector<int> c;
    Node(): c(2, -1) {}
    int &operator()(char nxt) {
        return c[nxt - '0'];
    }
};
vector<Node> trie;
int add(string &str, int now, int id) {
    //  cout<<now<<"now"<<id<<"id\n";
    if (id == -1) {
        id = tmp++;
    }

    if (now == str.size())
        return id;

    trie[id](str[now]) = add(str, now + 1, trie[id](str[now]));
    return id;
}
int search(string &str, int now, int id) {
    if (now == str.size())
        return 0;

    char c = '1' - str[now] + '0';

    if (trie[id](c) != -1) {
        return (1 << (D - now - 1)) + search(str, now + 1, trie[id](c));
    }

    return search(str, now + 1, trie[id](str[now]));
}
signed main() {
    quick
    trie.resize(2e6 + 7);
    int n;
    cin >> n;

    for (int i = 0; i < n; i++) {
        int a;
        cin >> a;
        s[i] = bin(a);
        //  cout<<s[i].size()<<"l\n";
        add(s[i], 0, 0);
    }//return 0;

    int ans = 0;

    for (int i = 0; i < n; i++) {
        int P = search(s[i], 0, 0);
        //  cout<<ans<<","<<P<<"P\n";
        ans = max(ans, P);
    }

    cout << ans << "\n";
    return 0;
}

int KMP(string a,string b){
	vector<int> f=buildf(b);
	int ans=0;
	int n=sz(a);
	int m=sz(b);
	int pos=0;
	rep(i,0,n-1){
		while(a[i]!=b[pos]&&pos){
			pos=f[pos-1];
		}
		if(a[i]==b[pos]) pos++;
		if(pos>=m){
			ans++;
			pos=f[pos-1];
		}
	}
	return ans;
}

vector<int> buildf(const string&p){
	int n=sz(p);
	vector<int> f(n);
	int pos=0;
	rep(i,1,n-1){
		while(p[i]!=p[pos]&&pos){
			pos=f[pos-1];
		}
		if(p[i]==p[pos]) pos++;
		f[i]=pos;
	}
	return f;
}

int KMP(string a,string b){
	vector<int> f=buildf(b);
	int ans=0;
	int n=sz(a);
	int m=sz(b);
	int pos=0;
	rep(i,0,n-1){
		while(a[i]!=b[pos]&&pos){
			pos=f[pos-1];
		}
		if(a[i]==b[pos]) pos++;
		if(pos>=m){
			ans++;
			pos=f[pos-1];
		}
	}
	return ans;
}

#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx,popcnt,sse4,abm")
#include<bits/stdc++.h>
#define int long long
#define quick ios::sync_with_stdio(0);cin.tie(0);
#define rep(x,a,b) for(int x=a;x<=b;x++)
#define repd(x,a,b) for(int x=a;x>=b;x--)
#define lowbit(x) (x&-x)
#define sz(x) (int)(x.size())
#define F first
#define S second
#define all(x) x.begin(),x.end()
#define mp make_pair
#define eb emplace_back
using namespace std;
typedef complex<int> P;
#define X real()
#define Y imag()
typedef pair<int,int> pii;
void debug(){
    cout<<"\n";
}
template <class T,class ... U >
void debug(T a, U ... b){
    cout<<a<<" ",debug(b...);
}
const int N=1e6+7;
const int INF=1e18;
pii operator + (pii a,pii b){
	return mp(a.F+b.F,a.S+b.S);
}
pii operator + (pii a, int b){
	return mp(a.F+b,a.S+b);
}
pii operator - (pii a,pii b){
	return mp(a.F-b.F,a.S-b.S);
}
pii operator * (pii a,int k){
	return mp(a.F*k,a.S*k);
}
pii operator * (pii a,pii b){
	return mp(a.F*b.F,a.S*b.S);
}
pii operator % (pii x,pii y){
	return mp(x.F%y.F,x.S%y.S);
}
bool operator == (pii a,pii b){
	return a.F==b.F&&a.S==b.S;
}
const pii Mod={1234567891,998244353};
int p;
pii pw[N];
pii pref[N];
pii suff[N];
pii q(int l,int r,bool pf){
	if(pf){
		return (pref[r]-pref[l-1]*pw[r-l+1]%Mod+Mod)%Mod;
	}
	return (suff[l]-suff[r+1]*pw[r-l+1]%Mod+Mod)%Mod;
}
bool ok(int posl,int pos,int pos2,int posr,int n){
	if(posl<=0||posr>n) {return false;}
	return q(posl,pos,true)==q(pos2,posr,false);
}
bool ok(int ln,int pos,int pos2,int n){
	return ok(pos-ln,pos,pos2,pos2+ln,n);
}
int qry(int pos,int pos2,int n){
	int l=0;
	int r=n+1;
	while(l+1<r){
		int mid=(l+r)>>1;
		if(ok(mid,pos,pos2,n)) l=mid;
		else r=mid;
	}
	return l;
}
signed main(){
	quick
	string s;
	cin>>s;
	int n=sz(s);
	s=' '+s;
	mt19937 rand;
	p=rand()%17+29;
	pw[0]=mp(1,1);
	rep(i,1,n){
		pw[i]=pw[i-1]*p%Mod;
	}
	rep(i,1,n){
		pref[i]=(pref[i-1]*p+(s[i]-'a'+1))%Mod;
	}
	repd(i,n,1){
		suff[i]=(suff[i+1]*p+(s[i]-'a'+1))%Mod;
	}
	int L,R;
	L=R=0;
	int ans=0;
	rep(i,1,n){
		int l1=qry(i,i,n);
		if(l1*2+1>ans){
			ans=l1*2+1;
			L=i-l1;
			R=i+l1;
		}
		if(i+1<=n&&s[i]==s[i+1]){
			int l2=qry(i,i+1,n);
			if(l2*2+2>ans){
				ans=l2*2+2;
				L=i-l2;
				R=i+l2+1;
			}
		}
	}
	rep(i,L,R) cout<<s[i];cout<<"\n";
 
	return 0;
}