int countPrimes(int n) { bool * IsPrime = (bool*)malloc(sizeof(bool) * n); for (int i = 0; i < n; i++) IsPrime[i] = true; for (int i = 2; i < n; i++) { if (IsPrime[i]) { for (int j = i + i; j < n; j += i) { IsPrime[j] = false; } } } int ans = 0; for (int i = 2; i < n; i++) if (IsPrime[i]) ans++; free(IsPrime); return ans; }