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77. Combinations

Backtracking

Problem - Combinations

Medium

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

Solutions

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class Solution:
    def combine(self, n: int, k: int) -> List[List[int]]:
        result = []
        temp = []

        def dfs(i: int):
            if len(temp) == k:
                result.append(temp[:])
                return
            if i > n:
                return
            temp.append(i)
            dfs(i + 1)
            temp.pop()
            dfs(i + 1)

        dfs(1)
        return result

Submission Stats:

  • Runtime: 135 ms (63.29%)
  • Memory: 59.7 MB (26.74%)