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.

難度：easy

題目：統計小于非負整數n的所有素數。

思路：參考素數篩選法。

Runtime: 11 ms, faster than 94.69% of Java online submissions for Count Primes.
Memory Usage: 35.9 MB, less than 21.43% of Java online submissions for Count Primes.

class Solution {public int countPrimes(int n) {boolean[] notPrimes = new boolean[n];int count = 0;for (int i = 2; i < n; i++) {if (!notPrimes[i]) {count++;for (int j = 2 * i; j < n; j += i) {notPrimes[j] = true;}}}return count;} }

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