Repeating substring

#include <bits/stdc++.h>
#include <fstream>
using namespace std;
#define PB push_back
#define N (1<<20)
#define QAQ {cout << "0";return 0; }
typedef long long ll;
const ll inf = 1000000007;
int t[N][20];

int f(int a, int b){
  int V = 0;
  //cout << "\n" << a << " " << b<< ": \n";
  for(int k=17; k>=0; --k){
  //  cout << t[a][k] << " " << t[b][k] << ", ";
    if(t[a][k] != t[b][k]) continue;
    V += (1<<k);
    a += (1<<k), b+= (1<<k);
  }
  return V;
}

int main(){
  ios::sync_with_stdio(0); cin.tie(0);
  string s; cin >> s;
	int n = s.size();
	for(int i=n; i<N; ++i) s+= "#";
	for(int i=0; i<N; ++i) t[i][0] = s[i];
	
	for(int b=0; b<=17; ++b){
	  vector<pair<ll,ll>> v;
	  for(int i=0; i<n; ++i){
	    v.PB({t[i][b] * inf + t[i+(1<<b)][b], i});
	  }
	  sort(v.begin(), v.end());
	  ll c=0, prev=0;
	  for(pair<ll, ll> p : v){
	    if(prev != p.first) prev = p.first, ++c;
	//    cout << s.substr(p.second, (1<<(b+1))) << " " << c<<  "\n";
	    t[p.second][b+1] = c;
	  }
	}
	vector<pair<ll, ll>> v;
	for(int i=0; i<n; ++i){
	  v.PB({t[i][17], i});
	}
	sort(v.begin(), v.end());
	int ml=0, V=0;
	for(int i=1; i<n; ++i){
	  int c = f(v[i-1].second, v[i].second);
	  if(c > ml) ml=c, V=v[i-1].second;
	}
	//cout << ml << " " << V << " ";
  for(int i=0; i<ml; ++i) cout << s[V+i];
}

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: ACCEPTED

Test 12

Verdict: UNKNOWN

Test 13

Verdict: UNKNOWN

Test 14

Verdict: UNKNOWN

Test 15

Verdict: UNKNOWN