204. Count Primes

Description:

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

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

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计算素数的方法要掌握，记得外围的判断标准是 n/2+1

class Solution {public:    int countPrimes(int n) {        vector<int> v1(n,1);        for(int i=2;i<n/2+1;++i)        {                        for(int k=i+i;k<n;k+=i)            {                if(v1[k]==1)                v1[k]=0;            }        }        int sum=0;        for(int i=2;i<n;++i)        sum+=v1[i];        return sum;    }};