78. Subsets Leetcode Java

Given a set of distinct integers, S, 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 S = [1,2,3], a solution is:

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

Solution:

Intuitively, the subset of set[n] is a set of all set[n-1] and set[n-1]"+" S[n] 

eg. for S=[1,2,3]

null: We start from empty set, set[0]={};

1:    set[1]={} and {1}={{}"+" 1};

1,2 : set[2]= set[1] + {set[1]"+"2} = {}, {1}, {2} and {1,2}

1,2,3: set[3] =set[2] + {set[2]"+"3} ={}, {1}, {2} ,{1,2}  {3},{1,3}, {2,3} and {1,2,3}

  public List<List<Integer>> subsets(int[] S) {  
    List<List<Integer>> res=new ArrayList<List<Integer>>();  
    if(S==null || S.length==0) return res;  
    Arrays.sort(S);  
    List<Integer> emptySet=new ArrayList<Integer>();  
    res.add(emptySet);  
    for(int i=0;i<S.length;i++){  
      int l=res.size();  
      for(int j=0;j<l;j++){  
        List<Integer> temp=new ArrayList<Integer>(res.get(j));  
        temp.add(S[i]);  
        res.add(temp);  
      }  
    }  
    return res;  
   }