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[n];    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;    }}