Phone Keypad Problem

Problem statement

Before the advent of QWERTY keyboards, texts and numbers were placed on the same key. For example, 2 had ABC and if we wanted to write anything starting with A, we needed to type key 2 once. If we wanted to type B, key 2 had to be pressed twice, and thrice for typing C. Below is a picture of such a keypad.

Given a keypad as shown in the diagram, and an n digit number, list all possible words by pressing these numbers. ...