820算法复试 Eratasthene 质数筛选

Eratasthene

学问之道无他，求其放心而巳矣

https://blog.csdn.net/qq_37653144/article/details/80470029

class Solution1 {
public:
    size_t countPrimes(size_t n) {
        bool *p = new bool[n+1];
        size_t i, j;
        for (i = 0; i <= n; ++i)
            p[i] = true;
        p[0] = p[1] = false;
        for (i = 2; i < n; ++i)
                if (p[i])
                    for (j = 2; i*j < n; ++j)
                        p[i*j] = false;
        size_t cnt = 0;
        for (i = 2; i < n; ++i)
            if (p[i])
                cnt++;
        delete []p;
        return cnt;
    }
};