add missing number finder and some other changes

main.go

@@ -1,5 +1,12 @@
 package main
 
+import (
+	"fmt"
+	"gotest/missing"
+)
+
 func main() {
 
+	fmt.Print(missing.FindNumber([]int{1, 3, 5, 2, 6, 7, -9, 8, 4}))
+
 }

missing/missing.go

@@ -0,0 +1,24 @@
+package missing
+
+import "fmt"
+
+func FindNumber(A []int) int {
+
+	var elements_sum int
+
+	for i := 0; i < len(A); i++ {
+
+		if A[i] < 0 || A[i] > 100000 {
+			fmt.Print("array has negative element or longer than expected")
+			return A[i]
+		} else {
+			elements_sum = elements_sum + A[i]
+		}
+
+	}
+
+	nums_sum := (len(A) + 1) * (len(A) + 2) / 2
+
+	return nums_sum - elements_sum
+
+}

missing/missing_test.go

@@ -0,0 +1 @@
+package missing_test

rotation/rotation.go

@@ -1,3 +1,4 @@
+// Циклическая ротация
 package rotation
 
 func Rotation(A []int, K int) []int {

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++ {
+	if len(A)%2 != 0 {
-		result = result ^ A[i]
+		for i := 0; i < len(A); i++ {
+			//xor use
+			result = result ^ A[i]
+		}
+	} else {
+		fmt.Println("the number of elements is even")
 	}
 
 	return result