Add reverse-integer solution

Leetcode id 7

Relates sashaaero#1

leet_7.py

@@ -0,0 +1,21 @@
+# https://leetcode.com/problems/reverse-integer
+# Runtime: 12 ms, faster than 96.32% of Python online submissions for Reverse Integer.
+
+class Solution:
+    def reverse(self, x):
+        """
+        :type x: int
+        :rtype: int
+        """
+        s = str(abs(x))
+
+        reversed = int(s[::-1])
+
+        if reversed > 2147483647:
+            return 0
+
+        return reversed if x > 0 else (reversed * -1)
+
+if __name__ == '__main__':
+    s = Solution()
+    assert s.reverse(123) == 321