78. Subsets

Given a set of distinct integers, nums, return all possible subsets.Note:
Elements in a subset must be in non-descending order.

The solution set must not contain duplicate subsets.

For example,
If nums =

[1,2,3]

, a solution is:

[
[3],
[1],
[2],
[1,2,3],
[1,3],
[2,3],
[1,2],
[]
]

解法：
一个集合大小为n，则集合的子集个数为 2^n
对集合进行编号，则(0,1,2,3,....2^n-1)
考察有2个元素的集合{1,2}，则有4个子集，{[],[1],[2],[1,2]}
其编号可用00,01,10,11表示,
则可以看出，子集编号二进制位中，相应为是1的就在该子集中，

vector<vector<int>> subsets(vector<int>& nums) {
int size=nums.size();
vector<vector<int>> res;
if(size==0)
return res;

long total=1<<size;
sort(nums.begin(),nums.end());

res.resize(total);

for(int i=0;i<size;++i){
long mask=1<<i;
for(long j=0;j<total;++j){
if(mask&j)
res[j].push_back(nums[i]);
}
}
return res;
}