contains-duplicate: refactor

- init map with size.
- check exists only in if statement.

Author: Invidam

contains-duplicate/invidam.go

@@ -1,10 +1,12 @@
 func containsDuplicate(nums []int) bool {
-	appeared := make(map[int]bool)
+	appeared := make(map[int]bool, len(nums))
 
 	for _, num := range nums {
-		if appeared[num] {
+		if _, found := appeared[num]; found {
 			return true
 		}
 		appeared[num] = true
 	}
-}
+
+	return false
+}