Create 0091-decode-ways.swift

Author: drxlx

swift/0091-decode-ways.swift

@@ -0,0 +1,18 @@
+class Solution {
+    func numDecodings(_ s: String) -> Int {
+        var s = Array(s)
+        var dp: [Int: Int] = [s.count: 1]
+        for i in stride(from: s.count - 1, to: -1, by: -1) {
+            if s[i] == "0" {
+                dp[i] = 0
+            } else {
+                dp[i] = dp[i + 1]
+            }
+
+            if i + 1 < s.count && (s[i] == "1" || s[i] == "2" && "0123456".contains(s[i + 1])) {
+                dp[i, default: 0] += dp[i + 2] ?? 0
+            }
+        }
+        return dp[0]!
+    }
+}