Merge pull request neetcode-gh#844 from notauserx/Create-91-Decode-Ways.cs

Create 91-Decode-Ways.cs

Author: Ahmad-A0

csharp/91-Decode-Ways.cs

@@ -0,0 +1,20 @@
+public class Solution {
+    public int NumDecodings(string s) {
+        List<int> dp = Enumerable.Repeat(1, s.Length + 1).ToList();
+        
+        for(var i = s.Length - 1; i >= 0; i--) {
+            if (s[i] == '0') 
+                dp[i] = 0;
+            else 
+                dp[i] = dp[i + 1];
+
+            if (i + 1 < s.Length && (
+                s[i] == '1' || 
+                (s[i] == '2' && "0123456".Contains(s[i + 1]))
+            ))
+                dp[i] += dp[i + 2];
+        }
+        
+        return dp[0];
+    }
+}