编写一个方法，返回某集合的所有子集

ArrayList<ArrayList<Integer>> getSubsets(ArrayList<Integer> set,int index)

{

ArrayList<ArrayList<Integer>> allsubsets;

if(set.size()==index)//终止条件，加入空集合

{

allsubsets= new ArrayList<ArrayList<Integer>>();

allsubsets.add(new ArrayList<Integer>());//空集合

}

else

{

allsubsets=getSubsets(set,index+1);

int item=set.get(index);

ArrayList<ArrayList<Integer>> moresubsets=new ArrayList<ArrayList<Integer>>();

for(ArrayList<Integer> subset:allsubsets)

{

ArrayList<Integer> newsubset=new ArrayList<Integer>();

newsubset.addAll(subset);

newsubset.add(item);

moresubsets.add(newsubset);

}

allsubsets.addAll(moresubsets);

}

return allsubsets;

}



ArrayList<ArrayList<Integer>> getSubsets2(ArrayList<Integer> set)

{

ArrayList<ArrayList<Integer>> allsubsets=new ArrayList<ArrayList<Integer>>();

int max=1<<set.size();//计算2^n;

for(int k=0;k<max;k++)

{

ArrayList<Integer> subset=convertIntToSet(k,set);

allsubsets.add(subset);

}

return allsubsets;

}

ArrayList<Integer> convertIntToSet(int x,ArrayList<Integer> set)

{

ArrayList<Integer> subset=new ArrayList<Integer>();

int index=0;

for(int k=x;k>0;k>>=1)

{

if((k & 1)==1)

{

subset.add(set.get(index));

}

index++;

}

return subset;

}