GOOGLE CODING INTERVIEW QUESTION - LETTER COMBINATIONS OF A PHONE NUMBER (LeetCode)

Letter Combinations of a Phone Number LeetCode coding solution. One of Google's most commonly asked interview questions according to LeetCode.

Coding Interviews Letter Combinations of a Phone Number (LeetCode) question and explanation.

This question is a commonly asked by the following companies: Google, Facebook, Amazon, Microsoft, Uber, JPMorgan, and Morgan Stanley.

Link to problem: https://leetcode.com/problems/letter-combinations-of-a-phone-number/description/

Problem description: Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.

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

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

Note: Although the above answer is in lexicographical order, your answer could be in any order you want.

Intuition behind solution: Create a mapping for digits to letters (i.e. 2 represent the letters a, b, and c). Call a recursive function to populate or letter combinations. Pass the recursive function our array list (result) that holds all our completed combinations, the numbers we're given, the mapping from numbers to letters, the current index we are on, and a string representing our current combination. Inside each recursive call, check if we have traversed all the digits we are given (index == digits.length()), if we have, add the current combination to our result and return to ensure we don't continue. Otherwise, get the letter mapping for the current digit we're on given by (mapping[digits.charAt(i) - '0'). Iterate through each possible letter that the current digit represents and at each iteration pass to the next recursive call letters[i], updating our representation of our current combination (current + letters[i]) and our index (index + 1). Once all the recursive calls terminate, we have successfully generated all possible letter combinations so we return result.

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