组合总数

public List<List<Integer>> combinationSum(int[] candidates,int target){
        if(candidates.length>0) {
            Arrays.sort(candidates);
            this.length=candidates.length;
            this.candidates=candidates;
            findCombinationSum(target,0,new Stack<Integer>());
        }
        return res;
    }

    private List<List<Integer>> res=new ArrayList<>();

    private Integer length;

    private int[] candidates;

    public void findCombinationSum(int residue,int start,Stack<Integer> stack){
        if(residue<0){
            return ;
        }
        if(residue==0){
            res.add(new ArrayList<>(stack));
            return;
        }
        for(int i=start;i<length&&residue-candidates[i]>=0;i++){
            stack.add(candidates[i]);
            findCombinationSum(residue-candidates[i],i,stack);
            stack.pop();
        }
    }