204. Count Primes(埃拉托色尼）

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.

class Solution {public:    int countPrimes(int n) {        if(n <= 2) return 0;        vector<int> prime(n+1, -1);        for(int i=2; i<sqrt(n); ++i){            if(prime[i] != 0){                int p = i + i;                while(p < n){                    prime[p] = 0;                    p += i;  //这里是+i,不要写走眼                }            }        }        int count = 0;        for(int i=2; i<n; ++i){            if(prime[i] != 0)                ++count;        }        return count;    }};