# 204.Count-Primes

## 204. Count Primes

## 题目地址

<https://leetcode.com/problems/count-primes/>

## 题目描述

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

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

## 代码

### Approach 1:

```java
class Solution {
  public int countPrimes(int n) {
        boolean[] notPrime = new boolean[n];
    int count = 0;
    for (int i = 2; i < n; i++) {
      if (notPrime[i] == false) {
        count++;
        for (int j = 2; i * j < n; j++) {
          notPrime[i * j] = true;
        }
      }
    }

    return count;
  }
}
```