README.md

77. Combinations

Given two integers n and k, return all possible combinations of k numbers out of 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

Solutions (Python)

1. Solution

class Solution:
    def combine(self, n: int, k: int) -> List[List[int]]:
        def foo(s: int, n: int, k: int) -> List[List[int]]:
            if k == 1:
                return [[i] for i in range(s, n + 1)]

            ret = []

            for i in range(s, n - k + 2):
                combs = foo(i + 1, n, k - 1)
                for comb in combs:
                    comb.append(i)
                ret.extend(combs)

            return ret

        return foo(1, n, k)