Permutation

Javascript Code

function perm(ar, l = 0, r = ar.length - 1, opts = []) {
  if (l === r) {
    opts.push(ar);
  }

  for (let i = l; i <= r; i++) {
    swap(ar, i, l)
    perm(ar.slice(0), l + 1, r, opts)
    swap(ar, l, i)
  }

  return opts;
}

function swap(arr, i, j) {
  let temp = arr[i];
  arr[i] = arr[j];
  arr[j] = temp;
}

Ruby Code

def perm(ar, l = 0, r = ar.length - 1, opts = [])
  if l == r
    opts.push(ar.dup)
  end

  (l..r).each do |i|
    ar[l], ar[i] = ar[i], ar[l]
    perm(ar, l + 1, r, opts)
    ar[i], ar[l] = ar[l], ar[i]
  end

  opts
end