{
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  {
   "cell_type": "markdown",
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   "source": [
    "# Euler Problem 24 \n",
    "\n",
    "A permutation is an ordered arrangement of objects. For example, 3124 is one possible permutation of the digits 1, 2, 3 and 4. If all of the permutations are listed numerically or alphabetically, we call it lexicographic order. The lexicographic permutations of 0, 1 and 2 are:\n",
    "\n",
    "012   021   102   120   201   210\n",
    "\n",
    "What is the millionth lexicographic permutation of the digits 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9?"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "I tried to solve this by thinking and failed to be hones. So we implement a generator and get the millionth element."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 19,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "from collections import deque\n",
    "\n",
    "def permutation_generator(xs):\n",
    "    if not xs:\n",
    "        yield deque()\n",
    "    r = []\n",
    "    for i in range(len(xs)):\n",
    "        x = xs[i]\n",
    "        ys = permutation_generator(xs[:i] + xs[i+1:])\n",
    "        for y in ys:\n",
    "            y.appendleft(x)\n",
    "            yield y\n",
    "    raise StopIteration\n",
    "\n",
    "assert(list(map(lambda s: \"\".join(s), permutation_generator(\"012\"))) == ['012', '021', '102', '120', '201', '210'])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {
    "collapsed": true
   },
   "source": [
    "All right, now we reuse the function to get the nth element from problem 7 and give ourselves a solution."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 26,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "2783915460\n"
     ]
    }
   ],
   "source": [
    "def get_nth(generator, n):\n",
    "    import itertools\n",
    "    return next(itertools.islice(generator, n - 1, n))\n",
    "\n",
    "ps = permutation_generator(\"0123456789\")\n",
    "s = int(\"\".join(get_nth(ps, 1000000)))\n",
    "assert(s == 2783915460)\n",
    "print(s)"
   ]
  }
 ],
 "metadata": {
  "completion_date": "Thu, 5 Nov 2015, 16:04",
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  "tags": [
   "permutation"
  ]
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