Leetcode: Count Primes

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Description:

Count the number of prime numbers less than a non-negative number, n.

数学的东西,有公式。最直观的检查到开方。还有很著名的算法:2的倍数,3的倍数,最大到开方一直检查下去。

class Solution {public:    int countPrimes(int n) {        vector<bool> primes(n, true);        for (int i = 2; i * i < n; ++i) {            if (!primes[i]) {                continue;            }            for (int j = i * i; j < n; j += i) {                primes[j] = false;            }        }                int count = 0;        for (int i = 2; i < n; ++i) {            if (primes[i]) {                ++count;            }        }                return count;    }};

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