Merge pull request neetcode-gh#1189 from voski/add-17-letter-combinat…

…ions-of-a-phone-number-for-ruby

add neetcode-gh#17 Letter Combinations of a Phone Number for ruby

ruby/17-Letter-Combinations-Of-A-Phone-Number.rb

@@ -0,0 +1,33 @@
+$MAP = {
+ 2 => %w(a b c),
+ 3 => %w(d e f),
+ 4 => %w(g h i),
+ 5 => %w(j k l),
+ 6 => %w(m n o),
+ 7 => %w(p q r s),
+ 8 => %w(t u v),
+ 9 => %w(w x y z) 
+}
+
+# @param {String} digits
+# @return {String[]}
+def letter_combinations(digits)
+ ans = []
+
+ recurse(digits, "", ans, 0) unless digits.empty?
+
+ ans
+end
+
+def recurse(digits, prefix, ans, i)
+ if digits.length == prefix.length
+ ans << prefix
+ return
+ end
+
+ digit = digits[i].to_i
+
+ $MAP[digit].each do |c|
+ recurse(digits, prefix + c, ans, i + 1)
+ end
+end