204. Count Primes

204. Count Primes

Description:

Count the number of prime numbers less than a non-negative number, n.
Analysis:
大家基本都会使用这个方法啦：https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
下面程序中vector的int类型改为bool应该会更好一些。
Source Code（C++）:

#include <iostream>
#include <vector>
#include <cmath>
using namespace std;

class Solution {
public:
    int countPrimes(int n) {
        if (n<2){
            return 0;
        }
        vector<int> v(n, 1);
        v.at(0)=0;v.at(1)=0;
        for(int i=2; i<sqrt((float)n); i++) {
            if(v.at(i)==1){
                for (int j=2; i*j<n; j++){
                    v.at(i*j)=0;
                }
            }           
        }
        int prime_counts=0;
        for (int i=0; i<n; i++){
            if (v.at(i)==1){
                prime_counts++;
            }
        }
        return prime_counts;
    }
};


int main() {
    Solution sol;
    cout << sol.countPrimes(0);
    cout << sol.countPrimes(100);
    return 0;
}