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;
    }
}