add solution of contains-duplicate

Author: Ujoonnee

contains-duplicate/Ujoonnee.java

@@ -0,0 +1,16 @@
+import java.util.HashSet;
+import java.util.Set;
+
+class Solution {
+    public boolean containsDuplicate(int[] nums) {
+        Set<Integer> numberSet = new HashSet<>();
+        for(int number : nums) {
+            if(numberSet.contains(number)) {
+                return true;
+            }
+
+            numberSet.add(number);
+        }
+        return false;
+    }
+}