Leetcode 204. Count Primes

easy

Count the number of prime numbers less than a non-negative number, n.

Example 1:

Input: n = 10
Output: 4
Explanation: There are 4 prime numbers less than 10, they are 2, 3, 5, 7.

Idea

• set every number up to n as “prime number”
• loop around and make any multiple of number as “not a prime number”