204. Count Primes

Description:

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

解法一

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;}