How should I practice Phone Keypad Letter Combinations?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Google onsite interview problems in FastPrep.

Is Phone Keypad Letter Combinations associated with Google?

FastPrep tags this as a Google onsite interview coding practice problem when the problem catalog has that company and stage context.

Problem · String

Phone Keypad Letter Combinations

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Problem statement

Given a string digits, return every letter combination that the digits could represent on a conventional telephone keypad.

2 maps to abc, 3 to def, 4 to ghi, 5 to jkl.
6 maps to mno, 7 to pqrs, 8 to tuv, and 9 to wxyz.

For this exercise, emit combinations in depth-first order, visiting each digit's mapped letters from left to right. Return an empty list for empty input.

Function

letterCombinations(digits: String) → List<String>

Examples

Example 1

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

Choose one of abc for the first position and one of def for the second, following left-to-right depth-first order.

Example 2

digits = ""return = []

There is no position to assign, so the exercise returns an empty list.

Constraints

0 <= digits.length <= 4
Every character in digits is between 2 and 9.