euler/ipython/EulerProblem018.ipynb

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   "source": [
    "# Euler Problem 18\n",
    "\n",
    "By starting at the top of the triangle below and moving to adjacent numbers on the row below, the maximum total from top to bottom is 23.\n",
    "\n",
    "~~~\n",
    "                    3\n",
    "                   7 4\n",
    "                  2 4 6\n",
    "                 8 5 9 3\n",
    "~~~\n",
    "\n",
    "That is, 3 + 7 + 4 + 9 = 23.\n",
    "\n",
    "Find the maximum total from top to bottom of the triangle below:\n",
    "\n",
    "~~~\n",
    "75\n",
    "95 64\n",
    "17 47 82\n",
    "18 35 87 10\n",
    "20 04 82 47 65\n",
    "19 01 23 75 03 34\n",
    "88 02 77 73 07 63 67\n",
    "99 65 04 28 06 16 70 92\n",
    "41 41 26 56 83 40 80 70 33\n",
    "41 48 72 33 47 32 37 16 94 29\n",
    "53 71 44 65 25 43 91 52 97 51 14\n",
    "70 11 33 28 77 73 17 78 39 68 17 57\n",
    "91 71 52 38 17 14 91 43 58 50 27 29 48\n",
    "63 66 04 68 89 53 67 30 73 16 69 87 40 31\n",
    "04 62 98 27 23 09 70 98 73 93 38 53 60 04 23\n",
    "~~~\n",
    "\n",
    "NOTE: As there are only 16384 routes, it is possible to solve this problem by trying every route. However, [Problem 67](https://projecteuler.net/problem=67), is the same challenge with a triangle containing one-hundred rows; it cannot be solved by brute force, and requires a clever method! ;o)"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "This is incredibly simple we simply from bottom to top choosing the higher value for each mini-tree.\n",
    "\n",
    "For example,\n",
    "\n",
    "~~~\n",
    "  95  64\n",
    "17  47  82\n",
    "~~~\n",
    "\n",
    "will become:\n",
    "\n",
    "~~~\n",
    " 142 146\n",
    "~~~"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "t = \"\"\"\n",
    "75\n",
    "95 64\n",
    "17 47 82\n",
    "18 35 87 10\n",
    "20 04 82 47 65\n",
    "19 01 23 75 03 34\n",
    "88 02 77 73 07 63 67\n",
    "99 65 04 28 06 16 70 92\n",
    "41 41 26 56 83 40 80 70 33\n",
    "41 48 72 33 47 32 37 16 94 29\n",
    "53 71 44 65 25 43 91 52 97 51 14\n",
    "70 11 33 28 77 73 17 78 39 68 17 57\n",
    "91 71 52 38 17 14 91 43 58 50 27 29 48\n",
    "63 66 04 68 89 53 67 30 73 16 69 87 40 31\n",
    "04 62 98 27 23 09 70 98 73 93 38 53 60 04 23\n",
    "\"\"\""
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "def reduce_rows(xs, ys):\n",
    "    \"\"\" xs is lower row and ys is upper row \"\"\"\n",
    "    assert(len(xs) == len(ys) + 1)\n",
    "    return [max([xs[i] + ys[i], xs[i + 1] + ys[i]]) for i in range(len(ys))]\n",
    "        \n",
    "assert(reduce_rows([17, 47, 82], [95, 64]) == [142, 146])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Okay, now all we have to do is the parsing and then a simple fold."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
   },
   "outputs": [],
   "source": [
    "xss = [list(map(int, xs.split())) for xs in t.split(\"\\n\") if xs]\n",
    "xss.reverse()\n",
    "from functools import reduce\n",
    "s = reduce(reduce_rows, xss[1:], xss[0])[0]\n",
    "assert(s == 1074)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Okay, let's put this into a nice function an then solve [problem 67](EulerProblem067) right away."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "1074\n"
     ]
    }
   ],
   "source": [
    "def find_greatest_path_sum_in_triangle_string(ts):\n",
    "    from functools import reduce\n",
    "    xss = [list(map(int, xs.split())) for xs in ts.split(\"\\n\") if xs]\n",
    "    xss.reverse()\n",
    "    r = lambda xs, ys: [max([xs[i] + ys[i], xs[i + 1] + ys[i]]) for i in range(len(ys))]\n",
    "    return reduce(r, xss[1:], xss[0])[0]\n",
    "\n",
    "print(find_greatest_path_sum_in_triangle_string(t))"
   ]
  }
 ],
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  "completion_date": "Thu, 4 Sep 2014, 20:38",
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