Add code to find prime number

Author: jbee37142

src/test/java/algorithm/basic/FindPrimeNumTest.java

@@ -0,0 +1,59 @@
+package algorithm.basic;
+
+import org.junit.Test;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertThat;
+
+public class FindPrimeNumTest {
+
+    /*
+        TASK
+        주어지는 수 이하의 소수 개수를 구한다.
+     */
+
+    @Test
+    public void test() {
+        assertThat(solution(-3), is(-1));
+        assertThat(solution(0), is(0));
+        assertThat(solution(1), is(0));
+        assertThat(solution(2), is(1));
+        assertThat(solution(3), is(2));
+        assertThat(solution(8), is(4));
+        assertThat(solution(12), is(5));
+        assertThat(solution(44), is(14));
+        //2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43
+    }
+
+    public int solution(int num) {
+        if (num < 0) {
+            return -1;
+        }
+        int[] checkList = new int[num + 1];
+
+        for (int i = 2; i <= num; i++) {
+            checkList[i] = i;
+        }
+        int baseNum = (int) Math.sqrt(num);
+
+        for (int i = 2; i <= baseNum; i++) {
+            if (checkList[i] == 0) {
+                continue;
+            }
+            for (int k = i; k <= num; k += i) {
+                if (checkList[k] != i && checkList[k] % i == 0) {
+                    checkList[k] = 0;
+                }
+            }
+        }
+
+        int count = 0;
+        for (int i = 0; i <= num; i++) {
+            if (checkList[i] != 0) {
+                count++;
+            }
+        }
+
+        return count;
+    }
+}