集合的所有子集

N个元素的集合的所有子集个数为2^N个。

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

ArrayList<ArrayList<Integer>> allsubsets;

if(index==0){//终止，加空集合

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

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

}

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(intem);

moresubsets.add(newsubset);

}

allsubsets.addAll(moresubsets);

}

return allsubsets;

}