【leetcode】Count Primes

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

 

class Solution {
public:
    int countPrimes(int n) {
        vector<bool> num(n-1,true);
        num[0]=false;
        int res=0;
        int 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;
    }
};