leetcode——204.——Count Primes

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

class Solution {public:    int countPrimes(int n) {     bool *isprime = new bool[n];    for (int i = 0; i < n; i++) {        isprime[i] = true;    }    for (int i = 2; i * i < n; i++) {        if (isprime[i]){        for (int j = i; j*i < n; j++) {            isprime[j*i] = false;        }        }    }    int cnt = 0;    for (int i = 2; i < n; i++) {        if (isprime[i]) cnt++;    }    return cnt;            }};