[leetcode] 77. Combinations

77. 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

class Solution(object):
    def combine(self, n, k):
        """
        :type n: int
        :type k: int
        :rtype: List[List[int]]
        """

        results = [] 
        def dfs(elements, start, k):
            if k == 0:
                results.append(elements[:])
            
            # 자신 이전의 모든 값을 고정하여 재귀 호출 
            for i in range(start, n + 1):
                elements.append(i)
                dfs(elements, i+1, k-1)
                elements.pop() 

        dfs([], 1, k)
        return results