LeetCode in Racket
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
(define (letter-combinations digits)
(let* ((letters (vector "" "" "abc" "def" "ghi" "jkl" "mno" "pqrs" "tuv" "wxyz"))
(letters2 (map string->list
(map (lambda (x) (vector-ref letters x))
(map (lambda (y) (string->number (make-string 1 y)))
(string->list digits))))))
(if (string=? "" digits)
'()
(map list->string (apply cartesian-product letters2)))))