add missing number finder and some other changes

unique/findunique.go

@@ -1,10 +1,18 @@
+// Чудные вхождения в массив
 package unique
 
+import "fmt"
+
 func FindUnique(A []int) int {
 	var result int
 
-	for i := 0; i < len(A); i++ {
-		result = result ^ A[i]
+	if len(A)%2 != 0 {
+		for i := 0; i < len(A); i++ {
+			//xor use
+			result = result ^ A[i]
+		}
+	} else {
+		fmt.Println("the number of elements is even")
 	}
 
 	return result