题目： 

 

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解析： 

 

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代码： 

 private List<List<Integer>> ret;
    private List<Integer> path;
    private int n,k;
    public List<List<Integer>> combine(int _n, int _k) {
        n = _n;
        k = _k;
        path = new ArrayList<>();
        ret = new ArrayList<>();
        dfs(1);
        return ret;
    } 

    private void dfs(int start){
        if(path.size() == k){
            ret.add(new ArrayList(path));
            return;
        } 

        for(int i = start; i <= n; i++){
            path.add(i);
            dfs(i+1);//剪枝：选过接着往后选

            path.remove(path.size()-1);//恢复现场
        }
    }