Count Primes

Description:

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

方法：利用哈希查表法进行排除。

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