impl Solution {
    pub fn combine(n: i32, k: i32) -> Vec<Vec<i32>> {
        let mut res = vec![];
        Solution::combine_0(n, k, &mut vec![], &mut res);
        return res;
    }

    fn combine_0(n: i32, k: i32, cur: &mut Vec<i32>, res: &mut Vec<Vec<i32>>) {
        if cur.len() == k as usize {
            res.push(cur.clone());
            return;
        }
        // 注意这个 s，保证了结果集是升序的
        let s = *cur.last().unwrap_or(&0) + 1;
        for i in s..=n {
            cur.push(i);
            Solution::combine_0(n, k, cur, res);
            cur.pop();
        }
    }
}