Project Euler Q20 - LO Man Hei

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  {
   "cell_type": "markdown",
   "id": "86e06153",
   "metadata": {},
   "source": [
    "### Q20\n",
    "Find the sum of the digits in the number 100!."
   ]
  },
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   "cell_type": "code",
   "execution_count": 1,
   "id": "16dac2f4-3864-43fc-a8a0-e99049bf2d12",
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   "source": [
    "def str_num_multiply(str_num_a, str_num_b):\n",
    "    len_a = len(str_num_a)\n",
    "    len_b = len(str_num_b)\n",
    "    result_str = [0]*(len_a+len_b)\n",
    "    carry = 0\n",
    "    for i in range(len_a)[::-1]:\n",
    "        for j in range(len_b)[::-1]:\n",
    "            temp_prod = result_str[j+i+1]+int(str_num_a[i])*int(str_num_b[j])+carry\n",
    "            result_str[j+i+1] = (temp_prod)%10\n",
    "            carry = temp_prod//10\n",
    "        if carry:\n",
    "            result_str[i+j] += carry\n",
    "            carry = 0\n",
    "    return \"\".join([str(i) for i in result_str]).lstrip(\"0\")\n",
    "\n",
    "def str_num_factorial(n):\n",
    "    return str_num_multiply(str(n),str_num_factorial(n-1)) if n else \"1\""
   ]
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    {
     "name": "stdout",
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     "text": [
      "CPU times: user 3.15 ms, sys: 420 µs, total: 3.57 ms\n",
      "Wall time: 3.57 ms\n"
     ]
    },
    {
     "data": {
      "text/plain": [
       "648"
      ]
     },
     "execution_count": 2,
     "metadata": {},
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   ],
   "source": [
    "%%time\n",
    "sum([int(c) for c in str_num_factorial(100)])"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "id": "61f65d34-9b29-4dc7-8365-508401a185d0",
   "metadata": {},
   "outputs": [],
   "source": [
    "def factorial(n):\n",
    "    return (n*factorial(n-1)) if n else 1"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "id": "3c5ccc8e-8a39-463f-b83e-53a961159812",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "CPU times: user 30 µs, sys: 4 µs, total: 34 µs\n",
      "Wall time: 35.8 µs\n"
     ]
    },
    {
     "data": {
      "text/plain": [
       "648"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "%%time\n",
    "sum([int(c) for c in str(factorial(100))])"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "bffadd80-d4f4-4a81-a63b-2ccb90748912",
   "metadata": {},
   "outputs": [],
   "source": []
  }
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