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class Solution {
public:
    int * flag;
    int countPrimes(int n) {
        flag = new int[n + 10];
        for (int i = 1;i < n;i++)
        {
            flag[i] = 0;
        }
        for (int i = 2;i < n;i++)
        {
            if (0 == flag[i])
            {
                flag[i] = 1;
                for (int j = 2;j * i < n; j++)
                {
                    flag[j * i] = -1;
                }
            }
        }
        int count = 0;
        for (int i = 1;i < n; i++)
        {
            if (1 == flag[i])
            {
                count++;
            }
        }
        return count;
    }
};