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  1. Time Complexity: Primality
  2. Discussions

Time Complexity: Primality

  • RodneyShag
     + 5 comments

    Java solution - passes 100% of test cases

    O(n^(1/2)) solution. We also skip even numbers for faster results.

    Alternatively, we could do preprocessing using the Sieve of Erasthenes and save the primes (up to a certain number) in an array or HashSet. Then determing if a number is prime would take O(1) time since it would be a simple lookup.

    public class Solution {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int p = scan.nextInt();
        while (p-- > 0) {
            int n = scan.nextInt();
            System.out.println(isPrime(n) ? "Prime" : "Not prime");
        }
        scan.close();
    }
    
    public static boolean isPrime(int n) {
        if (n < 2) {
            return false;
        } else if (n == 2) {
            return true;
        } else if (n % 2 == 0) {
            return false;
        }
        int sqrt = (int) Math.sqrt(n);
        for (int i = 2; i <= sqrt; i ++) {
            if (n % i == 0) {
                return false;
            }
        }
        return true;
    }
}

    From my HackerRank Java solutions.