Added alternative solution

Author: shichao-an

subsets/solution2.py

@@ -0,0 +1,18 @@
+class Solution:
+    # @param S, a list of integer
+    # @return a list of lists of integer
+    def subsets(self, S):
+        S.sort()
+        return self.subsets_aux(S)
+
+    def subsets_aux(self, S):
+        if not S:
+            return [[]]
+        else:
+            res = [[]]
+            for i, e in enumerate(S):
+                rest_subsets = self.subsets_aux(S[i + 1:])
+                for subset in rest_subsets:
+                    subset.insert(0, e)
+                res += rest_subsets
+            return res