permute = function (nums) {
  var result = [];
  backtrack(nums, result);
  return result;
}
https://www.youtube.com/watch?v=AfxHGNRtFac
https://www.geeksforgeeks.org/write-a-c-program-to-print-all-permutations-of-a-given-string/
https://leetcode.com/articles/permutations/
@param {number[]} nums
@return {number[][]}