Simplify FindNumberWhichAppearsOnce. Adjust PowerOfTwo javadoc

Fix some imports

Author: andrei-punko

src/main/java/by/andd3dfx/common/Permutations.java

@@ -3,7 +3,6 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
-import java.util.stream.Collectors;
 
 /**
  * <pre>

src/main/java/by/andd3dfx/numeric/FindNumberWhichAppearsOnce.java

@@ -36,9 +36,9 @@ public static int findSubOptimal(int[] numbers) {
     }
 
     public static int find(int[] numbers) {
-        int result = numbers[0];
-        for (int i = 1; i < numbers.length; i++) {
-            result ^= numbers[i];   // use fact that: ((X xor Y) xor Y) == X
+        int result = 0;
+        for (var number : numbers) {
+            result ^= number;   // use fact that: ((X xor Y) xor Y) == X
         }
         return result;
     }

src/main/java/by/andd3dfx/numeric/PowerOfTwo.java

@@ -23,6 +23,7 @@
  *
  * Input: n = 3
  * Output: false
+ * Explanation: There is no x where 2^x = 3.
  * </pre>
  */
 public class PowerOfTwo {

src/test/java/by/andd3dfx/numeric/PowerOfTwoTest.java

@@ -2,7 +2,8 @@
 
 import org.junit.Test;
 
-import static org.junit.Assert.*;
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
 
 public class PowerOfTwoTest {