LeetCode Count Primes

Description:

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

题意：给出一个数n,求小于n的素数的个数

思路：用 Sieve of Eratosthenes

代码如下：

class Solution
{public int countPrimes(int n){boolean[] isPrime = new boolean[n];for (int i = 2; i < n; i++){isPrime[i] = true;}int m = (int)Math.sqrt(n);for (int i = 2; i <= m; i++){if (!isPrime[i]) continue;for (int j = i * i; j < n; j += i){isPrime[j] = false;}}int count = 0;for (int i = 2; i < n; i++){if (isPrime[i]) count++;}return count;}
}

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