Combinations

class Solution:
    def __init__(self):
        self.path = []
        self.res = []

    def backtrack(self, n, k, startIndex):
        if len(self.path) == k:
            self.res.append(self.path.copy())
            return
        for i in range(startIndex, n):
            self.path.append(i)
            self.backtrack(n, k, i + 1)
            self.path.pop()

    def combine(self, n: int, k: int) -> List[List[int]]:
        self.backtrack(n + 1, k, 1)
        return self.res

Combinations

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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