solved #771, Python3

Contributors.md

@@ -29,3 +29,5 @@
 
 **Caitlin Genna** --> "CS Computer Science Graduate, Fordham University." --> [caitlingenna](https://github.com/caitlingenna)
 
+**Simone Corbo** --> "Physics student from Italy, Sapienza University" --> [simonecorbo99](https:://github.com/simonecorbo99)
+

Python3/Solution#771.py

@@ -0,0 +1,17 @@
+#-------Problem Description-------
+#
+# You're given strings J representing the types of stones that are jewels, and S representing the stones you have.  Each character in S is a type of stone you have.  You want to know how many of the stones you have are also jewels.
+#
+# The letters in J are guaranteed distinct, and all characters in J and S are letters. Letters are case sensitive, so "a" is considered a different type of stone from "A".
+#
+#
+
+class Solution:
+    def numJewelsInStones(self, J: str, S: str) -> int:
+        c = 0
+        for s in S:
+            if s in J:
+                c = c+1
+
+        return c
+