Language: C++

#pragma GCC target("avx2,avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <atcoder/all>
using namespace atcoder;
//using mint = modint998244353;
//using mint = modint1000000007;
#include <bits/stdc++.h>
using namespace std;
//#include <ext/pb_ds/assoc_container.hpp>
//#include <ext/pb_ds/tree_policy.hpp>
//using namespace __gnu_pbds;
//using i128 = __int128_t;
using ll = long long;
using ull = unsigned long long;
using pii = pair<int, int>;
using pll = pair<long long, long long>;
#define rep(i, n) for(int i = 0; i < (n); ++i)
#define all(x) (x).begin(),(x).end()
#define SZ(x) ((int)(x).size())
constexpr char ln = '\n';
template<class T1, class T2> inline bool chmax(T1 &a, T2 b) {if (a < b) {a = b; return true;} return false;}
template<class T1, class T2> inline bool chmin(T1 &a, T2 b) {if (a > b) {a = b; return true;} return false;}
inline int topbit(int x) {return x == 0 ? -1 : 31-__builtin_clz(x);}
inline int topbit(long long x) {return x == 0 ? -1 : 63-__builtin_clzll(x);}
inline int botbit(int x) {return x == 0 ? 32 : __builtin_ctz(x);}
inline int botbit(long long x) {return x == 0 ? 64 : __builtin_ctzll(x);}
inline int popcount(int x) {return __builtin_popcount(x);}
inline int popcount(long long x) {return __builtin_popcountll(x);}
inline int kthbit(long long x, int k) {return (x>>k)&1;}
inline void print() {cout << "\n";}
template<class T>
inline void print(const vector<T> &v) {
    for (auto itr = v.begin(); itr != v.end(); ++itr) cout << *itr << " ";
    print();
}
template<class T, class... Args>
inline void print(const T &x, const Args &... args) {
    cout << x << " ";
    print(args...);
}
#ifdef MINATO_LOCAL
#define dump(x) cerr << __LINE__ << " : " << #x << " = " << (x) << endl;
inline void debug() {cerr << endl;}
template<class T>
inline void debug(const vector<T> &v) {
    for (auto itr = v.begin(); itr != v.end(); ++itr) cerr << *itr << " ";
    debug();
}
template<class T, class... Args>
inline void debug(const T &x, const Args &... args) {
    cerr << x << " ";
    debug(args...);
}
#else
#define dump(x) void(0)
inline void debug() {}
template<class T> inline void debug(const vector<T> &v) {}
template<class T, class... Args> inline void debug(const T &x, const Args &... args) {}
#endif
struct Fast_ios {Fast_ios() {cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(20);};} fast_ios;
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

int main() {
    int K; cin >> K;
    string S; cin >> S;
    int N = S.size();

    int p = N/(K+1);//Sを長さがpとP+1の文字列に分ける
    int q = N%(K+1);//長さがp+1の文字列の個数

    auto SA = suffix_array(S);
    vector<int> SAinv(N);
    rep(i,N) SAinv[SA[i]] = i;
    if (q==0) {
        int MAX = 0;
        rep(i,N) {
            if (i%p==0) chmax(MAX,SAinv[i]);
        }
        cout << S.substr(SA[MAX],p) << ln;
        return 0;
    }

    auto check=[&](int mid) {
        vector<int> dp(N+1,-1e9);
        dp[0] = 0;
        rep(i,N) {
            if (N-i >= p) {
                chmax(dp[i+p],dp[i]);
            }
            if (N-i >= p+1) {
                if (SAinv[i]<=mid) chmax(dp[i+p+1],dp[i]+1);
            }
        }
        return dp[N] >= q;
    };

    int ok = N-1;
    int ng = -1;
    while (ok-ng>1) {
        int mid = (ok+ng)/2;
        (check(mid) ? ok : ng) = mid;
    }

    cout << S.substr(SA[ok],p+1) << ln;
}


Compiler output:
a.cc:4:10: fatal error: atcoder/all: No such file or directory
    4 | #include <atcoder/all>
      |          ^~~~~~~~~~~~~
compilation terminated.