Letter Combinations of a Phone Number

Tested Python solution for LeetCode 17 with 12 pytest cases. Generate a practice environment with lcpy.

LeetCode 17, Medium. Topics: Hash Table, String, Backtracking. View on LeetCode.

Generate this problem as a practice environment: tested reference solution, 12 parametrized pytest cases, and a playground notebook:

lcpy gen -n 17   # by problem number
lcpy gen -s letter_combinations_of_a_phone_number   # by problem name

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Problem

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.

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Examples

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

Input: digits = ""
Output: []

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

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Constraints

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

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Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

class Solution:
    # Time: O(4^n)
    # Space: O(4^n)
    def letter_combinations(self, digits: str) -> list[str]:
        if not digits:
            return []

        phone = {
            "2": "abc",
            "3": "def",
            "4": "ghi",
            "5": "jkl",
            "6": "mno",
            "7": "pqrs",
            "8": "tuv",
            "9": "wxyz",
        }

        result = []

        def backtrack(i: int, path: str) -> None:
            if i == len(digits):
                result.append(path)
                return

            for letter in phone[digits[i]]:
                backtrack(i + 1, path + letter)

        backtrack(0, "")
        return result

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Complexity

Time	Space
O(4^n)	O(4^n)

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Tags

Grind 75, Grind, NeetCode 150, NeetCode 250, NeetCode All.