diff --git a/missing_num.py b/missing_num.py
new file mode 100644
index 00000000..1d019ecc
--- /dev/null
+++ b/missing_num.py
@@ -0,0 +1,13 @@
+class Solution(object):
+    def missingNumber(self, nums):
+        """
+        :type nums: List[int]
+        :rtype: int
+        """
+        res = len(nums)
+        for i in range(len(nums)):
+            res += i - nums[i]
+        return res
+
+        # time complexity is O(n)
+        # space complexity is O(1)