Leetcode 77 Combinations

class Solution:
    def combine(self, n: int, k: int) -> List[List[int]]:
        re = [[]]
        for i in range(1, n+1):
            re += [_+[i] for _ in re if len(_)<k]
            if i-(n-k) > 0:
                re = [_ for _ in re if len(_) >= i-(n-k)]
        # print(f'from {n} get {k}: {re}')
        return re