diff --git a/contains-duplicate/hjeomdev.java b/contains-duplicate/hjeomdev.java
new file mode 100644
index 0000000000..bfb5ecfcc6
--- /dev/null
+++ b/contains-duplicate/hjeomdev.java
@@ -0,0 +1,17 @@
+import java.util.HashSet;
+import java.util.Set;
+
+class Solution {
+    public boolean containsDuplicate(int[] nums) {
+        Set set = new HashSet();
+        for (int num : nums) {
+            set.add(num);
+        }
+
+        if (set.size() != nums.length) {
+            return true;
+        } else {
+            return false;
+        }
+    }
+}
diff --git a/top-k-frequent-elements/hjeomdev.java b/top-k-frequent-elements/hjeomdev.java
new file mode 100644
index 0000000000..211942e64a
--- /dev/null
+++ b/top-k-frequent-elements/hjeomdev.java
@@ -0,0 +1,18 @@
+class Solution {
+    public int[] topKFrequent(int[] nums, int k) {
+        Map<Integer, Integer> map = new HashMap<>();
+
+        for (int num : nums) {
+            int current = map.getOrDefault(num, 0);
+            map.put(num, current + 1);
+        }
+
+        List<Integer> keySet = new ArrayList<>(map.keySet());
+        keySet.sort((o1, o2) -> map.get(o2).compareTo(map.get(o1)));
+
+        return keySet.subList(0, k)
+                .stream()
+                .mapToInt(Integer::intValue)
+                .toArray();
+    }
+}
diff --git a/two-sum/hjeomdev.java b/two-sum/hjeomdev.java
new file mode 100644
index 0000000000..a0815fae41
--- /dev/null
+++ b/two-sum/hjeomdev.java
@@ -0,0 +1,15 @@
+class Solution {
+    public int[] twoSum(int[] nums, int target) {
+        int[] answer = new int[2];
+        for (int i = 0; i < nums.length; i++) {
+            for (int j = i + 1; j < nums.length; j++) {
+                if (nums[i] + nums[j] == target) {
+                    answer[0] = i;
+                    answer[1] = j;
+                    break;
+                }
+            }
+        }
+        return answer;
+    }
+}