week 3: decode ways

Author: prograsshopper

decode-ways/prograsshopper.py

@@ -0,0 +1,19 @@
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        # sol 1: Time complexity O(n)
+        dp = {len(s): 1}
+
+        def dfs(start):
+            if start in dp:
+                return dp[start]
+            if s[start] == "0":
+                return 0
+            result = None
+            if start + 1 < len(s) and int(s[start:start+2]) < 27:
+                result = dfs(start+1) + dfs(start+2)
+            else:
+                result = dfs(start+1)
+            dp[start] = result
+            return result
+
+        return dfs(0)