decode-ways solution

decode-ways/sun912.py

@@ -0,0 +1,24 @@
+"""
+    TC: O(n)
+    SC: O(n)
+
+    DNF, read solutions by Dale...ㅠㅠ
+"""
+
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        memo = {len(s): 1}
+
+        def dfs(start):
+            if start in memo:
+                return memo[start]
+
+            if s[start] == "0":
+                return 0
+            if start + 1 < len(s) and int(s[start: start+2]) < 27:
+                memo[start] = dfs(start + 1) + dfs(start + 2)
+            else:
+                memo[start] = dfs(start + 1)
+            return memo[start]
+
+        return dfs(0)