Primer més petit que 100000.cc

#include <iostream>
#include <vector>
using namespace std;

int first_occurrence(int x, const vector<int>& v){
  if (v.size()==0) return -1;
  int inici = 0, final = v.size()-1;
  while (inici <= final) {
    int m = (inici + final)/2;
    if (v[m] < x) inici = m + 1;
    else final = m - 1;
  }
  if (final + 1 < v.size() and v[final + 1] == x){
    return final + 1;
  }
  return -1;
}

vector<int> sedas(int n){
  vector<int> B;
  vector<bool> A(n+1, true);
  for (int i = 2; i*i <= n; ++i){
    if (A[i]){
      B.push_back(i);
      for (int j = i*i; j < n+1; j+=i){
        A[j]=false;
      }
    }
  }
  return B;
}

int main(){
  int a;
  vector<int> u=sedas(1000001);
  while (cin >> a){
    if(first_occurrence(a, u) >= 0){
      cout << a << " es primer" << endl;
    }
    else{
      cout << a << " no es primer" << endl; 
    }
  }
}