Medium · trial division

Prime Check

Is n divisible by anything besides 1 and itself? Only test divisors up to √n — and stop the moment you find one. The √n bound is what makes it fast.

Problem

Input: one integer n (0 ≤ n ≤ 10⁶).

Output: Yes if n is prime, No otherwise (0 and 1 are not prime).

Sample:

Input	Output
`7`	`Yes`
`25`	`No`

Your solution

Program.cs — your solution

Custom input for "Run my input"

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Stuck? Hints

Hint 1 — why √n is enough

If n = a × b with a ≤ b, then a ≤ √n. A factor bigger than √n always pairs with one smaller — so testing up to i * i <= n covers everything.

Hint 2 — the edge cases

n < 2 → not prime (handle before the loop). Perfect squares like 25 need <= in i * i <= n, not < — otherwise 5×5 slips through. And break on the first divisor found.

Solution (try first!)

int n = int.Parse(Console.ReadLine());
if (n < 2) { Console.WriteLine("No"); return; }
bool prime = true;
for (int i = 2; i * i <= n; i++)
{
    if (n % i == 0) { prime = false; break; }
}
Console.WriteLine(prime ? "Yes" : "No");

return at top-level exits early — no else-branch needed for the n < 2 case. Scaling this to "count all primes below N" is a sieve — a good next challenge. Next: Fibonacci.