Solutions for Contains Duplicate #217

Author: dohyeon2

contains-duplicate/dohyeon2.java

@@ -0,0 +1,24 @@
+import java.util.HashSet;
+
+class Solution {
+    // The problem is to check if there are any duplicate elements in the array.
+    // So, I decided to use HashSet because it has O(1) time complexity for add and contains operations which are good to use for checking duplicates.
+    public boolean containsDuplicate(int[] nums) {
+        // the element type of the array is int, so create Integer HashSet
+        // O(n) space complexity
+        HashSet<Integer> exists = new HashSet<Integer>();
+        // the nums array has length up to 10^5, so use int type
+        // O(n) time complexity average
+        // worst case: O(n log n) if many elements hash to the same bucket
+        for(int i = 0; i < nums.length; i++){
+            int num = nums[i];
+            
+            if(exists.contains(num)){
+                return true;
+            }else{
+                exists.add(num);
+            }
+        }
+        return false;
+    }
+}