Add combination code

jbee37142 · jbee37142 · commit 0babf3d92722 · 2017-06-05T21:26:48.000+09:00
diff --git a/src/test/java/algorithm/basicMath/BasicCombination.java b/src/test/java/algorithm/basicMath/BasicCombination.java
@@ -1,7 +1,43 @@
 package algorithm.basicMath;
 
-/**
- * Created by Jbee on 2017. 6. 5..
- */
+import org.junit.Test;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.junit.Assert.assertThat;
+
 public class BasicCombination {
+
+    /*
+        TASK
+        n개의 서로 다른 원소 중 r개의 원소를 순서없이 선택하는 방법의 수를 구한다.
+     */
+
+    @Test
+    public void test() {
+        assertThat(getByRecursion(0, 0), is(1));
+        assertThat(getByRecursion(1, 0), is(1));
+        assertThat(getByRecursion(2, 1), is(2));
+        assertThat(getByRecursion(8, 3), is(56));
+
+        assertThat(getByDp(0, 0), is(1));
+        assertThat(getByDp(1, 0), is(1));
+        assertThat(getByDp(2, 1), is(2));
+        assertThat(getByDp(8, 3), is(56));
+    }
+
+    public int getByRecursion(int n, int r) {
+        if (r == 0 || n == r) {
+            return 1;
+        }
+        return getByRecursion(n - 1, r - 1) + getByRecursion(n - 1, r);
+    }
+
+    public int getByDp(int n, int r) {
+        int cache[][] = new int[10][10];
+        if (r == 0 || n == r) {
+            return 1;
+        }
+        if (cache[n][r] != 0) return cache[n][r];
+        return cache[n][r] = getByDp(n - 1, r - 1) + getByDp(n - 1, r);
+    }
 }
