Prime Number

An integer is prime if it has exactly two divisors: 1 and itself. Note that 1 is not a prime number.

// Author: github.com/lzl124631x
// Time: O(sqrt(N))
// Space: O(1)
bool isPrime(int n) {
    if (n < 2) return false;
    for (int d = 2; d * d <= n; ++d) {
        if (n % d == 0) return false;
    }
    return true;
}

Even-digit palindromes are not primes except 11

All even-digit palindromes, except for 11, are not primes because they are divisible by 11. For example, 2332 / 11 = 212

866. Prime Palindrome (Medium)