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    "https://leetcode.com/problems/letter-combinations-of-a-phone-number\n",
    "\n",
    "\n",
    "Runtime: 32 ms, faster than 61.61% of Python3 online submissions for Letter Combinations of a Phone Number.\n",
    "Memory Usage: 14.2 MB, less than 84.69% of Python3 online submissions for Letter Combinations of a Phone Number.\n",
    "\n",
    "\n",
    "\n",
    "```python\n",
    "import math\n",
    "\n",
    "class Solution:\n",
    "    def letterCombinations(self, digits: str) -> List[str]:\n",
    "        #7:54\n",
    "        if (digits == \"\"):\n",
    "            return []\n",
    "\n",
    "        num_to_abc_map = {\n",
    "            \"2\": \"abc\",\n",
    "            \"3\": \"def\",\n",
    "            \"4\": \"ghi\",\n",
    "            \"5\": \"jkl\",\n",
    "            \"6\": \"mno\",\n",
    "            \"7\": \"pqrs\",\n",
    "            \"8\": \"tuv\",\n",
    "            \"9\": \"wxyz\",\n",
    "        }\n",
    "        for key in num_to_abc_map.keys():\n",
    "            num_to_abc_map[key] = list(num_to_abc_map[key])\n",
    "            \n",
    "        possible_set_list = [num_to_abc_map[digit] for digit in digits]\n",
    "        length = math.prod([len(i) for i in possible_set_list])\n",
    "        \n",
    "        def loop(index, result):\n",
    "            if index == 0 and len(result) == 0:\n",
    "                return loop(index+1, possible_set_list[index])\n",
    "            elif len(result) == length:\n",
    "                return result\n",
    "            \n",
    "            new_list = []\n",
    "            for num1 in result:\n",
    "                for num2 in possible_set_list[index]:\n",
    "                    new_list.append(num1+num2)\n",
    "\n",
    "            return loop(index+1, new_list)\n",
    "            \n",
    "        return loop(0, [])\n",
    "        #8:44\n",
    "```"
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