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class Solution {
public:
    int countPrimes(int n) {
        bool isPrime[n+1];
        for(int i = 2; i < n; i++)
            isPrime[i] = true;

        for(int i = 2; i * i < n; 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;
    }
};