花花酱 LeetCode 1952. Three Divisors

Given an integer n, return true if n has exactly three positive divisors. Otherwise, return false.

An integer m is a divisor of n if there exists an integer k such that n = k * m.

Example 1:

Input: n = 2
Output: false
Explantion: 2 has only two divisors: 1 and 2.

Example 2:

Input: n = 4
Output: true
Explantion: 4 has three divisors: 1, 2, and 4.

Constraints:

• 1 <= n <= 104

Solution: Enumerate divisors.

Time complexity: O(n)
Space complexity: O(1)

Optimization

Only need to enumerate divisors up to sqrt(n). Special handle for the d * d == n case.

Time complexity: O(sqrt(n))
Space complexity: O(1)