issue #16 204-Done

sowon-dev · sowon-dev · commit c947bfeb15a2 · 2021-01-25T23:21:31.000+09:00
diff --git a/src/leetcodeEasyLevel/_0204countPrimes.java b/src/leetcodeEasyLevel/_0204countPrimes.java
@@ -1,16 +1,38 @@
 package leetcodeEasyLevel;
 
 public class _0204countPrimes {
+
   //prime numbers &#51593; &#49548;&#49688;&#47484; &#52286;&#45716; &#47928;&#51228;
   public static int countPrimes(int n) {
-    for(int i =0; i<n; i++){
-      System.out.println(" "+n);
+    boolean[] nums = new boolean[n];
+    int primeCount = 0;
+    // 1&#48512;&#53552; n&#44620;&#51648;&#51032; &#49707;&#51088;&#47484; true&#47196; &#52488;&#44592;&#54868;&#54620;&#45796;.
+    for (int i = 0; i < n; i++) {
+      nums[i] = true;
+    }
+
+    //&#51228;&#44273;&#51060; &#44032;&#45733;&#54620; &#49707;&#51088;&#45716; false&#47196; &#48320;&#44221;&#54620;&#45796;.
+    for (int i = 2; i * i < n; i++) {
+      //false&#47484; true &#48148;&#44984;&#51648; &#50506;&#46020;&#47197; &#51312;&#44148;&#51012; &#44148;&#45796;.
+      if (nums[i] == true) {
+        //&#48176;&#49688;&#46308;&#51008; &#45796; false&#47196; &#48320;&#44221;&#54620;&#45796;.
+        for (int j = 2; j * i < n; j++) {
+          nums[j * i] = false;
+        }
+      }
+    }
+
+    //true&#47564; &#49464;&#44592;
+    for (int i = 2; i < n; i++) {
+      if (nums[i] == true) {
+        primeCount++;
+      }
     }
-    return 0;
+    return primeCount;
   }
 
   public static void main(String[] args) {
-    int  n = 10; //=> 4
+    int n = 10; //=> 4
     System.out.println(countPrimes(n));
   }
 }
