Cant find the fix for my program at LeetCode Challenge of PrimeNumbers

This'll also pass through:

public class Solution {
    public static final int countPrimes(int n) {
        // mapping for if the number is divisible by prime numbers, which would make that number a composite number
        boolean[] notPrime = new boolean[n];
        // counting prime numbers
        int count = 0;

        for (int i = 2; i < n; i++) {
            // If the index of notPrime would be false, we have a prime number, we go through the if, otherwise we continue
            if (notPrime[i] == false) {
                // Increment the number of prime numbers
                count++;

                // Look into future numbers
                for (int j = 2; i * j < n; j++) {
                    // find composite numbers and set their indices to true
                    notPrime[i * j] = true;
                }
            }
        }

        return count;
    }
}

References

• For additional details, you can see the Discussion Board. There are plenty of accepted solutions with a variety of languages and explanations, efficient algorithms, as well as asymptotic time/space complexity analysis^{1, 2} in there.

• Sieve of Eratosthenes

• YouTube 1

• YouTube 2

You got confused with the input/output part: you don't need any scanner to do this, just:

 class Solution 
 {
      public static int countPrimes(int n) 
       {
          int sum = 0;
           int a[] = new int [n];
           for(int i = 0; i < a.length; i++) {
                a[i] = i;
                int  cont = 0;
            
                for(int y = 1; y< a.length; y++) {
                    if(a[i] % y == 0) {
                       cont++;
                     }
                 }
          if (cont == 2) {
              sum++;
          }
        }
       return sum; //this is the output
    }
        
     public static void main(String args[])
    ´{
        countPrimes(10); //this is the input
     }
}

Proof:

Et voilá. LeetCode accepts the input (10) and the output (4). That's all you need :)

Your answer gets a scanner object which is not needed therefore you could remove it.

You also create an array which will decrease performance, which I recommend not using as you do not need to display the prime numbers but only keep track of them.

public static int countPrimes(int n) {

    int sum=0;

    for(int i = n; i > 1; i-- ){
        
        int count = 0; //Keep track of the number of primes.
        
        for(int j = 2; j < i; j++){
            if(i % j == 0){
                count++;
            }
        }
        
        if(count==0) {
            sum++;
        }
      
    }

    return sum;

    }