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class Solution {

// ttps://en.wikipedia.org/wiki/Sieve_of_Eratosthenes#Algorithm_complexity

public int countPrimes(int n) {

boolean[] isPrime = new boolean[n];

int count = 0;

Arrays.fill(isPrime, true);

for (int i = 2; i < n; i++) {

if (i * i >= n)

break;

if (!isPrime[i])

continue;

for (int j = i * i; j < n; j += i)

isPrime[j] = false;

}

for (int i = 2; i < n; i++)

if (isPrime[i])

count++;

return count;

}

}