testcase "[1,2]" fail in OJ but success in my local machine, see my codes:

```
class Solution:
# @param S, a list of integer
# @return a list of lists of integer
sets = []
s_set = []
def subsets(self, S):
self._subsets(sorted(S), 0)
return Solution.sets
def _subsets(self, S, level):
if level == len(S):
return Solution.sets.append(Solution.s_set[:])
self._subsets(S, level+1)
Solution.s_set.append(S[level])
self._subsets(S, level+1)
Solution.s_set.pop()
```

From OJ:

```
Input: [1,2]
Output: [[],[0],[],[2],[1],[1,2]]
Expected: [[],[1],[2],[1,2]]
```