add #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