Source: https://leetcode.com/problems/combinations/

Given two integers n and k, return all possible combinations of k numbers out of the range [1, n]. You may return the answer in any order.

/**
* @param {number} n
* @param {number} k
* @return {number[][]}
*/
// O(N!/(N-k)!k!) time and space for output
var combine = function(n, k) {
 const combinations = [];
 const combinationsHelper = (index, currentCombo) => {
   if (currentCombo.length === k) {
     combinations.push([...currentCombo]);
     return;
   }
   
   for (let i = index; i <= n; i++) {
     currentCombo.push(i);
     combinationsHelper(i + 1, currentCombo);
     currentCombo.pop();
   }
 }
 
 combinationsHelper(1, []);
 return combinations;
};