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Coding Test

Created 2022-05-17 / Updated 2022-05-17

Programmers / 소수 찾기

Problem

Solution 1

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import java.util.ArrayList;

class Solution {
    public int solution(int n) {
        ArrayList<Integer> primeList = new ArrayList<>(1000000);
        int answer = 0;
        
        // Find prime number
        for (int i = 2; i <= n; i++) {
            boolean isPrime = true;
            for (int j = 0; j < primeList.size(); j++) {
                if (i % primeList.get(j) == 0) {
                    isPrime = false;
                    break;
                }
            }
            
            if (isPrime) {
            	primeList.add(i);
            	answer++;
            }
        }
        
        return answer;
    }
}
Solution 1

Solution 2

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import java.util.Arrays;

class Solution {
    public int solution(int n) {
        // Init isPrime array
        boolean[] isPrime = new boolean[n+1];
        Arrays.fill(isPrime, true);
        isPrime[0] = false; // 0 is not prime
        isPrime[1] = false; // 1 is not prime
        
        // Find prime number
        for (int i = 2; i <= n; i++) {
            if (isPrime[i]) {
                for (int j = 2*i; j <= n; j = j + i) {
                    isPrime[j] = false;
                }
            }
        }
        
        // Get prime number count
        int count = 0;
        for (int i = 0; i < isPrime.length; i++) {
            if (isPrime[i]) {
                count++;
            }
        }
        return count;
    }
}
Solution 2