写一个关于全排列，全组合的函数

/**
 * @param {number[]} nums
 * @return {number[][]}
 */
var permute = function (nums) {
  let result = [];
  let used = Array.from({ length: nums.length }).fill(false);
  function search(collection, used) {
    if (collection.length === nums.length) {
      result.push(collection);
      return;
    }
    for (let i = 0; i < nums.length; i++) {
      if (used[i] === false) {
        used[i] = true;
        search(collection.concat(nums[i]), used.slice(0));
        used[i] = false; // 重置状态
      }
    }
    collection = null;
    used = null;
  }
  search([], used);
  return result;
};

var combine = function (n, k) {
  let result = [];
  function find(collection, from) {
    if (collection.length === k) {
      result.push(collection);
      return;
    }
    for (let i = from; i <= n; i++) {
      find(collection.concat(i), i + 1);
    }
  }
  find([], 1);
  return result;
};