solve problem

Author: delight010

decode-ways/delight010.swift

@@ -0,0 +1,19 @@
+class Solution {
+    func numDecodings(_ s: String) -> Int {
+        var array = Array(s)
+        var dp: [Int: Int] = [array.count: 1]
+        for i in stride(from: array.count - 1, to: -1, by: -1) {
+            if array[i] == "0" {
+                dp[i] = 0
+            } else {
+                dp[i] = dp[i + 1]
+            }
+
+            if i + 1 < array.count && (array[i] == "1" || array[i] == "2" && "0123456".contains(array[i + 1])) {
+                dp[i, default: 0] += dp[i + 2] ?? 0
+            }
+        }
+        return dp[0]!
+    }
+}
+