题目描述：

第一次提交：

class Solution:
    def combine(self, n: int, k: int) -> List[List[int]]:
        res = []
        def backtrack(i,temp_list):
            if len(temp_list)==k:
                res.append(temp_list)
            for j in range(i,n+1):
                backtrack(j+1,temp_list+[j])
        backtrack(1,[])
        return res

 方法二：

class Solution:
    def combine(self, n: int, k: int) -> List[List[int]]:
        nums = list(range(1, k + 1)) + [n + 1]
        
        output, j = [], 0
        while j < k:
            output.append(nums[:k])
            j = 0 
            while j < k and nums[j + 1] == nums[j] + 1: 
                nums[j] = j + 1 
                j += 1 
            nums[j] += 1 
        return output