Letter Combinations of a Phone Number | CodeTutor

Letter Combinations of a Phone Number

Medium

Strings Hash Tables Recursion Backtracking

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.

Digit	Letters
2	a, b, c
3	d, e, f
4	g, h, i
5	j, k, l
6	m, n, o
7	p, q, r, s
8	t, u, v
9	w, x, y, z

Constraints

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

Examples

Input: digits = "23"

Output: ["ad","ae","af","bd","be","bf","cd","ce","cf"]

Digit 2 maps to 'abc' and digit 3 maps to 'def'. Combining each letter from 2 with each letter from 3 gives 9 combinations.

Input: digits = ""

Output: []

Empty input returns an empty list.

Input: digits = "2"

Output: ["a","b","c"]

Digit 2 maps to 'abc', so we return all three letters.

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