77. 组合

class Solution {

    List<List<Integer>> res;
    List<Integer> path;
    int n;
    int k;
    public List<List<Integer>> combine(int n, int k) {
        res = new ArrayList<>();
        path = new ArrayList<>();
        this.n = n;
        this.k = k;
        dfs(1);
        return res;
    }

    private void dfs(int index) {
        if (path.size() == k) {
            res.add(new ArrayList<>(path));
            return;
        }
        for (int i = index; i <= n - (k - path.size()) + 1; i++) {
            path.add(i);
            dfs(i + 1);
            path.remove(path.size() - 1);
        }
    }
}