added code for reverse integer problem

0007/README.md

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+# Reverse Integer
+
+### Problem statemen
+Given a signed 32-bit integer x, return x with its digits reversed. If reversing x causes the value to go outside the signed 32-bit integer range [-231, 231 - 1], then return 0.
+
+**Assume the environment does not allow you to store 64-bit integers (signed or unsigned).**

0007/reverse_integer.py

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+class Solution:
+    def reverse(self, x):
+        """
+        :type x: int
+        :rtype: int
+        """
+        sign = [1,-1][x < 0]
+        rst = sign * int(str(abs(x))[::-1])
+        return rst if -(2**31)-1 < rst < 2**31 else 0