LeetCode----204. Count Primes(Java)

 package countPrimes204;
 /*
  * Description:
  * Count the number of prime numbers less than a non-negative number, n.
  */
 public class Solution {
     //Time Limit Exceeded
     /*
     public static int countPrimes(int n) {
         int number=0;
         for (int i=0;i<n;i++)
             if(IsPrime(i)==true)
                 number++;
         return number;
     }
      public static boolean IsPrime(int n){
            if (n <= 3)
                 return n > 1;
            else if (n%2==0||n%3==0)
                return false;
            else{
                for(int i=2;i<=Math.sqrt(n);i++){
                    if(n%i == 0)
                        return false;
                }
            }
            return true;
        }
       */

     public static int countPrimes(int n) {
         //boolean default is false
         boolean[] IsPrime=new boolean[n];
         int numPrime=0;
         for (int i=2;i<n;i++){
             if (IsPrime[i-1]==false){
                 numPrime++;
                 for(int j=2;i*j<n;j++)
                     IsPrime[i*j-1]=true;
             }
         }
         return numPrime;
     }
     public static void main(String[] args) {
         // TODO Auto-generated method stub
         System.out.println(countPrimes(15000000));
         boolean[] IsPrime=new boolean[2];
         System.out.println(IsPrime[0]);
     }
     //reference
     public int countPrimes2(int n) {
         boolean[] notPrime = new boolean[n];
         int count = 0;
         for (int i = 2; i < n; i++) {
             if (notPrime[i] == false) {
                 count++;
                 for (int j = 2; i*j < n; j++) {
                     notPrime[i*j] = true;
                 }
             }
         }
         return count;
     }
 }