Count Primes

Description:

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

class Solution {public:    int countPrimes(int n) {        if (n<=2) return 0;        vector<bool> passed(n, false);        int sum = 1;        int upper = sqrt(n);        for (int i=3; i<n; i+=2) {            if (!passed[i]) {                sum++;                //avoid overflow                if (i>upper) continue;                for (int j=i*i; j<n; j+=i) {                    passed[j] = true;                }            }        }        return sum;    }};