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;

    }
};