答案

public class Solution {
    int[] candidates;
    public List<List<Integer>> combinationSum(int index ,int target)
    {
        List<List<Integer>> result=new LinkedList<List<Integer>>();
        if(index>=candidates.length||candidates[index]>target)
        {
            return result;
        }
        List<Integer> p=new LinkedList<Integer>();
        while(target>=0)
        {
            if(target==0)
            {
                result.add(p);
                break;
            }
            List<List<Integer>> next=combinationSum(index+1,target);
            if(next.size()>0)
            {
                for(List<Integer> list:next)
                {
                    List<Integer> pList=new LinkedList<Integer>();
                    pList.addAll(p);
                    pList.addAll(list);
                    result.add(pList);
                }
            }
            p.add(candidates[index]);
            target-=candidates[index];
        }
        return result;
        
    }
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> result=new LinkedList<List<Integer>>();
        if(candidates==null||target<=0)
        {
            return result;
        }
        Arrays.sort(candidates);
        this.candidates=candidates;
        return combinationSum(0,target);
    }
}