diff --git a/reverse-bits/river20s.java b/reverse-bits/river20s.java
new file mode 100644
index 000000000..76a976fe9
--- /dev/null
+++ b/reverse-bits/river20s.java
@@ -0,0 +1,22 @@
+public class Solution {
+    /* T.C = O(1)
+     * S.C = O(1)
+     */
+    public int reverseBits(int n) {
+        // Set the output to 0
+        int output = 0;
+        // Repeat 32 times 
+        for (int i = 0; i < 32; i++) {
+            // Shift the output value one space to the left to make room for the new bit
+            output <<= 1;
+            // '&' operation to get the rightmost bit and add it to the output 
+            output = output | (n & 1);
+            // Discard the rightmost bit of the 'n'
+            n = n >> 1;
+        }
+
+        return output;
+        
+    }
+}
+
diff --git a/two-sum/river20s.java b/two-sum/river20s.java
new file mode 100644
index 000000000..621660e25
--- /dev/null
+++ b/two-sum/river20s.java
@@ -0,0 +1,24 @@
+/*
+ * T.C: O(n) -> 배열 nums를 한 번 순회
+ * S.C: O(n) → 최대 n개의 요소가 저장됨
+ */
+import java.util.HashMap;
+
+class Solution {
+    public int[] twoSum(int[] nums, int target) {
+
+        HashMap<Integer, Integer> map = new HashMap<>();
+        for  (int i = 0; i < nums.length; i++) {
+            int k = target - nums[i];
+
+            if (map.containsKey(k)) {
+                return new int[] { map.get(k), i };
+            }
+
+            map.put(nums[i], i);
+        }
+        throw new IllegalArgumentException("exception handling for java compilation");
+        
+    }
+}
+