Add a new method for 007_Reverse_Integer.py (#80)

* Add a new method for 007 reverse integer

Contributed by @ROMEEZHOU

python/007_Reverse_Integer.py

@@ -1,17 +1,34 @@
 class Solution:
     def reverse(self, x):
         # https://leetcode.com/problems/reverse-integer/
-        flag = True if x < 0 else False
-        if flag:
+#        flag = True if x < 0 else False
+#       if flag:
+#           x = -x
+#       x = str(x)[::-1]
+
+#       if flag:
+#           x = "-" + x
+
+#       value = 2 ** 31
+#       x = int(x)
+#       if -value <= x < value:
+#           return x
+#       return 0
+
+        is_neg = False
+        if x < 0:
             x = -x
-        x = str(x)[::-1]
+            is_neg = True
 
-        if flag:
-            x = "-" + x
+        res = 0
+        while x > 0:
+            res *= 10
+            res += x % 10
+            x //= 10
+        if is_neg:
+            res = -res
 
-        value = 2 ** 31
-        x = int(x)
-        if -value <= x < value:
-            return x
-        return 0
-
+        if res < -2**31 or res > 2**31-1:
+            return 0
+        return res
+