README.md

Letter Combinations of a Phone Number

Medium

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.

A mapping of digits to letters (just like on the telephone buttons) is given below. Note that 1 does not map to any letters.

Example 1:

Input: digits = "23"
Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]

Example 2:

Input: digits = ""
Output: []

Example 3:

Input: digits = "2"
Output: ["a","b","c"]

Constraints:

• 0 <= digits.length <= 4
• digits[i] is a digit in the range ['2', '9'].

Problem can be found in here!

Solution: Recursion

class Solution:
    def letterCombinations(self, digits: str) -> List[str]:
        self.digit2letter = {"2": "abc", "3": "def", "4": "ghi", "5": "jkl",
                             "6": "mno", "7": "pqrs", "8": "tuv", "9": "wxyz"}
        output_list = []
        if digits:
            self.dfs(output_list, digits, 0, [])
        return output_list

    def dfs(self, output_list: List[str], digits: str, index: int, tempt: List[int]):
        if len(digits) == index:
            output_list.append("".join(tempt))
            return

        for letter in self.digit2letter[digits[index]]:
            tempt.append(letter)
            self.dfs(output_list, digits, index+1, tempt)
            tempt.pop()

Time Complexity: $O(4^n)$, Space Complexity: $O(n)$, where n is the length of array $digits$.