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77.combinations.py
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48 lines (42 loc) · 1.11 KB
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# Tag: Backtracking
# Time: O(k * C(n, k))
# Space: O(K)
# Ref: -
# Note: -
# Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n].
# You may return the answer in any order.
#
# Example 1:
#
# Input: n = 4, k = 2
# Output: [[1,2],[1,3],[1,4],[2,3],[2,4],[3,4]]
# Explanation: There are 4 choose 2 = 6 total combinations.
# Note that combinations are unordered, i.e., [1,2] and [2,1] are considered to be the same combination.
#
# Example 2:
#
# Input: n = 1, k = 1
# Output: [[1]]
# Explanation: There is 1 choose 1 = 1 total combination.
#
#
# Constraints:
#
# 1 <= n <= 20
# 1 <= k <= n
#
#
class Solution:
def combine(self, n: int, k: int) -> List[List[int]]:
res = []
tmp = []
self.dfs(n, k, 1, tmp, res)
return res
def dfs(self, n: int, k:int, start: int, tmp: list, ans: List[List[int]]):
if len(tmp) == k:
ans.append(tmp.copy())
return
for i in range(start, n + 1):
tmp.append(i)
self.dfs(n, k, i + 1, tmp, ans)
tmp.pop()