Submission

Score: 90

User: KanemiyaKana

Problemset: จำนวนเฉพาะก่อนหน้า

Language: cpp

Time: 0.030 second

Submitted On: 2026-09-30 18:32:44

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

vector<int> sieve(int thelast)
{
    vector<bool> primenum(thelast + 1, true);
    primenum[0] = false;
    primenum[1] = false;
    for (int num = 2; num * num <= thelast; num++) // just illiterating from 2 to the sqr of the final number
    {
        if (primenum[num] == true) // means if the current number IS a prime
        {
            for (int i = num * num; i <= thelast; i += num) // discarding multiples of the number that has been checked as prime in the above if, example if the number that has been checked true is 2 then this is simply just telling you to start fro 2*2 which is 4 and add 2 each time to discard the multiples of it until the point of the last number
            {
                primenum[i] = false; // discarded non-primes
            }
        }
    }
    vector<int> returnnum;
    for (int p = thelast; p > 0; p--)
    {
        if (primenum[p])
        {
            returnnum.push_back(p);
        }
        if (returnnum.size() == 5) // size instead of length for vector
        {
            break;
        }
    }
    return returnnum;
}

int main()
{
    int n;
    cin >> n;
    if (n < 0)
    {
        return 0;
    }
    else
    {
        vector<int> output = sieve(n);
        for (int i = int(output.size()) - 1; i >= 0; i--)
        {
            cout << output[i] << " ";
        }
    }
}