Leetcode 204. Count Primes

[easy]

Count Primes - LeetCode

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.

Example 2:

Input: n = 0
Output: 0

Example 3:

Input: n = 1
Output: 0

Constraints:

• 0 <= n <= 5 * 106

[Think]

1. create a boolean array and default them as true
2. change them to false if there you can multiple them

[Trick]

1. if already change to false, don’t need to change and scan again
2. for (int j = i*i; j < n; j+= i)

[Second]

1. use notPrime