leetcode: Letter Combinations of a Phone Number
Given a digit string, 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.
Input:Digit string "23" Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].
Although the above answer is in lexicographical order, your answer could be in any order you want.
Solution and Precautions:
A very standard backtracking (DFS) problem, no any tricks, but note the actually test data does not test on 0 or 1, these two digits. It is understandable since it is fuzzy on the letters mapped by these two digits.
Tips and Divergent thinking:
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