add decode ways problem

Author: sagivo

decode_ways.rb

@@ -0,0 +1,16 @@
+# problem: https://leetcode.com/problems/decode-ways/description/
+# @param {String} s
+# @return {Integer}
+def num_decodings(s)
+    return 0 if s.empty? or s.start_with? '0'
+    close = far = 1
+
+    (2..s.size).each do |i|
+        cur = 0
+        cur = close if s[i-1] != '0'
+        cur += far if s[i-2] == '1' or (s[i-2] == '2' and s[i-1].to_i <= 6)
+        far = close
+        close = cur
+    end
+    return close
+end