LeetCode—204 Count Primes

思路：建立一个布尔数组，从2开始读取这个数组，读到false进去维护下，并给返回的值+1

public class Solution {
public int countPrimes(int n) {
boolean[] flag=new boolean
;
int ans=0;
for (int i = 2; i < flag.length; i++) {
if (!flag[i]) {
ans++;
for (int j = i; j < flag.length; j+=i) {
flag[j]=true;
}
}
}
return ans;
}
}