add missing number finder and some other changes

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