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：

def combine(self, n: int, k: int) -> List[List[int]]:
	nums = [_ for _ in range(1, n + 1)]
	path, result = [], []
	self.dfs(nums, 0, k, path, result)
	return result

def dfs(self, nums, st, k, path, result):
	if k == 0:
		result.append(list(path))
		return

	for i in range(st, len(nums)):
		path.append(nums[i])
		self.dfs(nums, i + 1, k - 1, path, result)
		path.pop()

算法分析：

时间复杂度为O(2k)，空间复杂度O(n)