{
 "cells": [
  {
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   "source": [
    "# Euler Problem 20\n",
    "\n",
    "n! means n × (n − 1) × ... × 3 × 2 × 1\n",
    "\n",
    "For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,\n",
    "and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.\n",
    "\n",
    "Find the sum of the digits in the number 100!"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Write our own factorial implementation and get the solution."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def factorial(n):\n",
    "    assert(n > 0)\n",
    "    if n == 1:\n",
    "        return 1\n",
    "    else:\n",
    "        return n * factorial(n - 1)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "648\n"
     ]
    }
   ],
   "source": [
    "f100 = factorial(100)\n",
    "print(sum(map(int, str(f100))))\n",
    "assert(sum(map(int, str(f100))) == 648)"
   ]
  }
 ],
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  "completion_date": "Fri, 5 Sep 2014, 10:42",
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