# Leetcode 17 - Letter combinations of a phone number

Note:

• This is a backtracking problem.
• Not like other combination sum questions. You are not just iterating a single string, instead, you are iterating many strings in responding to every digit of digits.
• Three steps to do backtracking problems
• The backtracking params are current index of digits and digits.
• The termination condition is index === digits.length
• The logic of iterating on each level is the most important part
• Get the string according to index.
• Do backtracking.

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.

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