diff --git a/missing-number/hi-rachel.py b/missing-number/hi-rachel.py
new file mode 100644
index 000000000..90debd532
--- /dev/null
+++ b/missing-number/hi-rachel.py
@@ -0,0 +1,38 @@
+"""
+TC: O(n)
+SC: O(n)
+"""
+from typing import List
+
+
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        num_set = set(nums)
+        for i in range(len(nums)):
+            if i not in num_set:
+                return i
+        return len(nums)
+
+
+"""
+TC: O(n log n)
+SC: O(1)
+"""
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        nums.sort()
+        for idx, num in enumerate(nums):
+            if idx != num:
+                return idx
+        return len(nums)
+
+
+"""
+TC: O(n)
+SC: O(1)
+"""
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        expected_sum = len(nums) * (len(nums) + 1) // 2 
+        actual_sum = sum(nums)
+        return expected_sum - actual_sum