Letter Combinations of a Phone Number

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Problem

Given a string of digits from 2-9, return all possible letter combinations the number could represent, based on the standard telephone keypad letter mapping.

Examples

Example 1

Input:digits = "23"

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

Example 2

Input:digits = ""

Output:[]

Constraints

0 <= digits.length <= 4

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How to approach it: the Backtracking pattern

Explore a decision one choice at a time, undo it if it doesn't lead anywhere, and try the next option. It's a systematic way to generate every valid combination, permutation, or arrangement.

Look for this pattern when

The problem asks for "all possible" combinations, subsets, permutations, or valid arrangements.
A greedy or direct-formula approach doesn't work because you genuinely need to explore multiple branches.

Read the full Backtracking guide →

Video walkthroughs

Original problem on LeetCode ↗