LeetCode-in-All
17. Letter Combinations of a Phone Number
Medium
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.
Example 1:
Input: digits = “23”
Output: [“ad”,”ae”,”af”,”bd”,”be”,”bf”,”cd”,”ce”,”cf”]
Example 2:
Input: digits = “”
Output: []
Example 3:
Input: digits = “2”
Output: [“a”,”b”,”c”]
Constraints:
0 <= digits.length <= 4digits[i]is a digit in the range['2', '9'].
Solution
-spec letter_combinations(Digits :: unicode:unicode_binary()) -> [unicode:unicode_binary()].
letter_combinations(Digits) ->
Res = lists:foldl(fun(Digit, Acc) ->
case Digit of
<<"2">> ->
[<<Accbin/binary, Byte/binary>> || Byte <- [<<A>> || <<A:8>> <= <<"abc">>], Accbin <- Acc];
<<"3">> ->
[<<Accbin/binary, Byte/binary>> || Byte <- [<<A>> || <<A:8>> <= <<"def">>], Accbin <- Acc];
<<"4">> ->
[<<Accbin/binary, Byte/binary>> || Byte <- [<<A>> || <<A:8>> <= <<"ghi">>], Accbin <- Acc];
<<"5">> ->
[<<Accbin/binary, Byte/binary>> || Byte <- [<<A>> || <<A:8>> <= <<"jkl">>], Accbin <- Acc];
<<"6">> ->
[<<Accbin/binary, Byte/binary>> || Byte <- [<<A>> || <<A:8>> <= <<"mno">>], Accbin <- Acc];
<<"7">> ->
[<<Accbin/binary, Byte/binary>> || Byte <- [<<A>> || <<A:8>> <= <<"pqrs">>], Accbin <- Acc];
<<"8">> ->
[<<Accbin/binary, Byte/binary>> || Byte <- [<<A>> || <<A:8>> <= <<"tuv">>], Accbin <- Acc];
<<"9">> ->
[<<Accbin/binary, Byte/binary>> || Byte <- [<<A>> || <<A:8>> <= <<"wxyz">>], Accbin <- Acc]
end
end, [<<>>], [<<B>> || <<B:8>> <= Digits]),
case Res of
[<<>>] -> [];
_ -> Res
end.