22_combine.py

'''
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.'''
from typing import List
class Solution:
  def combine(self, n: int, k: int) -> List[List[int]]:
    ans=[]
    def backtrack(n,idx,k,temp):
      if len(temp)==k:
        ans.append(temp[:])
        return 
      for i in range(idx,n+1):
        temp.append(i)
        backtrack(n,i+1,k,temp)
        temp.pop()
    backtrack(n,1,k,[])
    return ans