77. Combinations

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

JavaScript

/**
 * @param {number} n
 * @param {number} k
 * @return {number[][]}
 */
var combine = function(n, k) {
  var result = []
  helper(n, k, 1, [], result)
  return result
}

function helper(n, k, m, nums, result) {
  if (nums.length === k) {
    result.push(nums.slice())
    return
  }

  for (let i = m; i <= n; i++) {
    nums.push(i)
    helper(n, k, i + 1, nums, result)
    nums.pop()
  }
}

Python

class Solution(object):

    def combine(self, n, k):
        """
        :type n: int
        :type k: int
        :rtype: List[List[int]]
        """
        result = []
        self.helper(n, k, 1, [], result)
        return result

    def helper(self, n, k, m, nums, result):
        if len(nums) == k:
            result.append(nums[:])
            return
        for i in range(m, n + 1):
            nums.append(i)
            self.helper(n, k, i + 1, nums, result)
            nums.pop()