Repeating substring

#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("arch=skylake")

#include <iostream>
#include <string>
#include <algorithm>
#include <vector>

using namespace std;
typedef long long ll;

int n;
string str;

vector<int> suffix(string s) {
	int N = n+256;
	vector<int> sa(n), ra(n);
	for (int i = 0; i < n; ++i) {
		sa[i] = i;
		ra[i] = (int)s[i];
	}
	for (int k = 0; k < n; k?k*=2:k++) {
		vector<int> nsa(sa), nra(n), cnt(N);
		for (int i = 0; i < n; ++i) nsa[i] = (nsa[i]-k+n)%n;
		for (int i = 0; i < n; ++i) cnt[ra[i]]++;
		for (int i = 1; i < N; ++i) cnt[i] += cnt[i-1];
		for (int i = n-1; i >= 0; --i) sa[--cnt[ra[nsa[i]]]] = nsa[i];
		int r = 0;
		for (int i = 1; i < n; ++i) {
			if (ra[sa[i]] != ra[sa[i-1]]) r++;
			else if (ra[(sa[i]+k)%n] != ra[(sa[i-1]+k)%n]) r++;
			nra[sa[i]] = r;
		}
		ra = nra;
	}
	return sa;
}

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	cin >> str;
	n = str.length();
	vector<int> sa = suffix(str);
	int mxlen = 0;
	int mxi = -1;
	for (int i = 0; i < n-1; ++i) {
		int a = sa[i];
		int b = sa[i+1];
		int clen = 0;
		while (a < n && b < n && str[a] == str[b]) {
			a++;
			b++;
			clen++;
		}
		if (clen > mxlen) {
			mxlen = clen;
			mxi = sa[i];
		}
		if (clen == n) break;
	}
	if (mxlen == 0) {
		cout << "\n";
		return 0;
	}
	string res = str.substr(mxi, mxlen);
	cout << res << "\n";
	return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: TIME LIMIT EXCEEDED

Test 12

Verdict: UNKNOWN

Test 13

Verdict: UNKNOWN

Test 14

Verdict: UNKNOWN

Test 15

Verdict: UNKNOWN