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//结果
    List<List<Integer>> result = new ArrayList<>();
    public List<List<Integer>> combine(int n , int k){
        backTracking(n,k,1);

        return result;
    }

    //小集合
    List<Integer> path = new ArrayList<>();
    private void backTracking(int n, int k,int startNum) {
        if(path.size() == k){
            result.add(new ArrayList<>(path));
            return;
        }

        for (int i = startNum;i <= n - (k - path.size()) + 1; i++) {
            path.add(i);
            backTracking(n,k    ,i + 1);
            path.remove(path.size() - 1);
        }
    }