Determine if a string can form an additive sequence, where each number is the sum of the two preceding numbers. The goal is to check if such a sequence exists starting from the beginning of the string.

Given a string of digits, return all possible letter combinations that the number could represent, using the typical phone keypad mapping. The goal is to generate all valid combinations in any order.

A self-dividing number is divisible by every digit it contains. This problem asks you to find all the self-dividing numbers within a given range.