Update f-exuan21.java

HashSet -> Set 수정

Author: f-exuan21

contains-duplicate/f-exuan21.java

@@ -3,7 +3,7 @@
 
 class Solution {
     public boolean containsDuplicate(int[] nums) {
-        HashSet<Integer> set = new HashSet<>();
+        Set<Integer> set = new HashSet<>();
         for(int i : nums) {
             if(set.contains(i)) {
                 return true;