204. Count Primes

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

public class Solution {      public int countPrimes(int n) {       //2,3,5,7,11,13,17        //20  5       //init check  n       boolean[] a = new boolean[n];       for(int i=2; i*i<n; i++) {          if(!a[i]) {              for(int j=i; i*j<n; j++) {                  a[i*j] = true;              }          }       }       int c=0;       for(int i=2; i<n; i++) {           if(a[i] == false) ++c;       }       return c;      }  }  //素数不能被比它小的整数整除, 建一个boolean 数组, 从2开始, 把其倍数小于N的都删掉.//注意 inner loop从i开始, 比i小的会在以前就被check过.