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

Posted 2020-08-09

 

 1 package countPrimes204;
 2 /*
 3  * Description:
 4  * Count the number of prime numbers less than a non-negative number, n.
 5  */
 6 public class Solution {
 7     //Time Limit Exceeded
 8     /*
 9     public static int countPrimes(int n) {
10         int number=0;
11         for (int i=0;i<n;i++)
12             if(IsPrime(i)==true)
13                 number++;
14         return number;
15     }
16      public static boolean IsPrime(int n){
17            if (n <= 3) 
18                 return n > 1;
19            else if (n%2==0||n%3==0)
20                return false;
21            else{
22                for(int i=2;i<=Math.sqrt(n);i++){
23                    if(n%i == 0)
24                        return false;
25                }
26            }
27            return true;
28        }
29       */
30     
31     public static int countPrimes(int n) {
32         //boolean default is false
33         boolean[] IsPrime=new boolean[n];
34         int numPrime=0;
35         for (int i=2;i<n;i++){
36             if (IsPrime[i-1]==false){
37                 numPrime++;
38                 for(int j=2;i*j<n;j++)
39                     IsPrime[i*j-1]=true;
40             }
41         }
42         return numPrime;
43     }
44     public static void main(String[] args) {
45         // TODO Auto-generated method stub
46         System.out.println(countPrimes(15000000));
47         boolean[] IsPrime=new boolean[2];
48         System.out.println(IsPrime[0]);
49     }
50     //reference
51     public int countPrimes2(int n) {
52         boolean[] notPrime = new boolean[n];
53         int count = 0;
54         for (int i = 2; i < n; i++) {
55             if (notPrime[i] == false) {
56                 count++;
57                 for (int j = 2; i*j < n; j++) {
58                     notPrime[i*j] = true;
59                 }
60             }
61         }
62         return count;
63     }
64 }