77. Combinations

Problem Link

Description

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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

Solution

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Python3

class Solution:
    def combine(self, n: int, k: int) -> List[List[int]]:
        res = []
 
        for mask in range(1, 1 << n):
            if mask.bit_count() != k: continue
 
            curr = []
            for j in range(n):
                if mask & (1 << j):
                    curr.append(j + 1)
            
            res.append(curr)
        
        return res