updated at 2022. 12. 29. 오전 4:23:06

programmers/소수 찾기/solution.java

@@ -1,18 +1,30 @@
-// [문제 링크]: https://school.programmers.co.kr/learn/courses/30/lessons/12921
+// [문제 링크]: https://school.programmers.co.kr/learn/courses/30/lessons/42839
+
+import java.util.HashSet;
 
 class Solution {
-    public int solution(int n) {
+    public int solution(String numbers) {
+        HashSet<Integer> hs = new HashSet<>();
+        per("",numbers,hs);
         int answer = 0;
-        for(int a = 2; a<=n; a++){
-            int t = 0;
-            for(int b=2; b<=Math.sqrt(a); b++){
-                if(a%b==0) {
-                    t = 1;
-                    break;
-                }
-            }
-            if(t == 0) answer+= 1;
+        while(hs.iterator().hasNext()){
+            int a = hs.iterator().next();
+            hs.remove(a);
+            if(a==2) answer++;
+            if(a%2!=0 && prime(a)) answer++;
         }
         return answer;
     }
+    private boolean prime(int n){
+        if(n==0||n==1) return false;
+        for(int i=3; i<=(int)Math.sqrt(n); i+=2){
+            if(n%i==0) return false;
+        }
+        return true;
+    }
+    private void per(String pre, String str, HashSet<Integer> hs){
+        int len = str.length();
+        if(!pre.equals("")) hs.add(Integer.valueOf(pre));
+        for (int i = 0; i < len; i++) per(pre + str.charAt(i), str.substring(0,i) + str.substring(i+1, len),hs);
+    }
 }