204. Count Primes

题目：

Description:

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

思路：

本题主要采用埃拉托斯特尼筛法（详情请戳sieve of Eratosthenes）

代码：

class Solution {public:    int countPrimes(int n) {        vector<bool> num(n - 1, true);        num[0] = false;        int res = 0, limit = sqrt(n);        for (int i = 2; i <= limit; ++i) {            if (num[i - 1]) {                for (int j = i * i; j < n; j += i) {                    num[j - 1] = false;                }            }        }        for (int j = 0; j < n - 1; ++j) {            if (num[j]) ++res;        }        return res;    }};