LeetCode:Subsets I

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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],  []]

开始想法以给定数组为字典序进行排序，给定一个子序列之后求下一个序列，最终得到全部序列：

public class Solution {    ArrayList<Integer> nextSubSet(ArrayList<Integer> in,int [] A)    {        int len=A.length;        int i,j,size=in.size();        if(len==size)return null;        int num=in.get(0);        ArrayList<Integer> out=new ArrayList<Integer>();        if(num==A[len-size]){            for(i=0;i<size+1;i++)out.add(A[i]);            return out;        }        Iterator<Integer> it=in.iterator();        while(it.hasNext())out.add(it.next());        i=0;        while(out.get(size-1-i)==A[len-1-i])i++;        i=size-1-i;        num=out.get(i);        for(j=0;j<len;j++)if(A[j]==num)break;        out.set(i,A[j+1]);        i=i+1;        j=j+2;for(;i<size;i++,j++)out.set(i,A[j]);        return out;    }    public ArrayList<ArrayList<Integer>> subsets(int[] S) {        ArrayList<ArrayList<Integer>> list=new ArrayList<ArrayList<Integer>>();        int len=S.length;        if(len==0)return list;        Arrays.sort(S);        ArrayList<Integer> innerList=new ArrayList<Integer>();        list.add(innerList);        innerList=new ArrayList<Integer>();        innerList.add(S[0]);        while(innerList!=null)        {            list.add(innerList);            innerList=nextSubSet(innerList,S);                    }        return list;    }}

之后想到可以用递归直接来做，实质上是DFS：

public class Solution {      ArrayList<ArrayList<Integer>> list=new ArrayList<ArrayList<Integer>>();      ArrayList<Integer> copy(ArrayList<Integer> in)      {                      ArrayList<Integer> out=new ArrayList<Integer>();          Iterator<Integer> it=in.iterator();          while(it.hasNext())out.add(it.next());          return out;      }    public void dfs(int [] A,int index,ArrayList<Integer> in)    {           if(index==A.length){            ArrayList<Integer> out=copy(in);            list.add(out);            return;            }        in.add(A[index]);        dfs(A,index+1,in);        in.remove(in.size()-1);        dfs(A,index+1,in);    }    public ArrayList<ArrayList<Integer>> subsets(int[] S) {         ArrayList<Integer> in=new ArrayList<Integer>();        int len=S.length;        Arrays.sort(S);        dfs(S,0,in);        return list;    }}

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