leetcode: 递归 77. Combinations

题目

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Example:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

思路

组合分两部分：

        1、[1,n-1] 内的K个数 

        2、[1,n-1] 内的K-1个数 + n

    def combine(self, n: int, k: int) -> List[List[int]]:
        if n == 0 or k == 0:
            return []
        elif k == 1:
            ans = []
            for i in range(n):
                ans.append([i+1])
            return ans
        
        ans = self.combine(n-1,k)
        second = self.combine(n-1,k-1)
        for item in second:
            ans.append(item+[n])
        return ans