add contains-duplicate py3 soln

0217_contains-duplicate/python3/solution.py

@@ -0,0 +1,11 @@
+class Solution:
+    def containsDuplicate(self, nums: List[int]) -> bool:
+        counts = {}
+        
+        for num in nums:
+            counts[num] = counts.get(num, 0) + 1
+            
+            if counts[num] > 1:
+                return True
+        
+        return False