组合问题77

class :
    def helper(self, nums, k):
        n = len(nums)
        
        if n < k:
            return []
        if n == k:
            return [nums]
        if k == 1:
            return [[i] for i in nums]
        res = []
        # 递归过程
        for i in range(n):
            start = [nums[i]]
            # 相当于在i之后的nums中选取k-1个数
            sub_res = [start + s for s in self.helper(nums[i + 1:], k - 1)]
            res.extend(sub_res)
        return res
  # n中选k个
    def combine(self, n, k):
        # 上一个函数参数为nums
        nums = [i + 1 for i in range(n)]
        res = self.helper(nums, k)
        return res


a = [1, 2, 3, 4, 5]
print(Solution().combine(5, 3))