77. 组合

class Solution:
    def _combine(self, n, k, start, nums, res):
        if len(nums) == k:
            res.append(nums.copy())
            return
        
        for i in range(start, n+1):
            nums.append(i)
            self._combine(n, k, i+1, nums, res)
            nums.pop()
            
    def combine(self, n, k):
        """
        :type n: int
        :type k: int
        :rtype: List[List[int]]
        """
        res = list()
        if n <= 0 or k <= 0 or n < k:
            return res
        nums = list()
        self._combine(n, k, 1, nums, res)
        return res