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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
# @param {String} digits
# @return {String[]}
def letter_combinations(digits)
return [] if digits.empty?
letters = ["", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"]
ans = []
find_combinations(0, digits, letters, "", ans)
ans
end
private
def find_combinations(start, nums, letters, curr, ans)
if curr.length == nums.length
ans << curr
return
end
(start...nums.length).each do |i|
n = nums[i].to_i
letters[n].each_char do |ch|
find_combinations(i + 1, nums, letters, curr + ch, ans)
end
end
end