LeetCode 077 Combinations

LeetCode

Given two integers n and k, return all possible combinations of k numbers out of the range [1, n].

You may return the answer in any order.

Example 1:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

Example 2:

Input: n = 1, k = 1
Output: [[1]]

Constraints:

1 <= n <= 20 1 <= k <= n

题目大意：

求大小为k的所有可能组合

解题思路：

用组合模板，先排序

解题步骤：

N/A

注意事项：

1. 引入k作为模板API中的target，k为0作为终止条件。

Python代码：

# char dfs (type 2 linear) Leetcode 77 Given two integers n and k, return all possible combinations of k numbers out of the range [1, n]
def combine(self, n: int, k: int) -> List[List[int]]:
	nums = [i + 1 for i in range(n)]
	path, res = [], []
	self.dfs2(nums, 0, k, [], res)
	return res

def dfs2(self, nums, start, k, path, res):
	if len(path) == k:
		res.append(list(path))
		return
	for i in range(start, len(nums)):
		path.append(nums[i])
		self.dfs2(nums, i + 1, k, path, res)
		path.pop()

算法分析：

时间复杂度为O(kCkn)，解大小乘以path长度, 空间复杂度O(n)