204-Count-Primes.cpp

class Solution {
public:
    int countPrimes(int n) {
        if (n <= 2)
            return 0;

        vector<bool> arr(n, true);
        arr[0] = arr[1] = false;
        for (int i = 2; i * i < n; ++i) {
            if (arr[i]) {
                for (int j = i * i; j < n; j += i) {
                    arr[j] = false;
                }
            }
        }

        int count = 0;
        for (int i = 2; i < n; ++i) {
            if (arr[i]) {
                count++;
            }
        }

        return count;
    }
};