decode ways solution

Author: yuseok89

decode-ways/yuseok89.py

@@ -0,0 +1,29 @@
+# TC: O(N)
+# SC: O(N)
+class Solution:
+
+    memo = []
+
+    def rec(self, idx, s, n):
+        if self.memo[idx] == -1:
+
+            cnt = 0
+
+            if int(s[idx]) != 0:
+                cnt += self.rec(idx + 1, s, n)
+
+            if idx + 1 < n:
+                if int(s[idx]) != 0 and int(s[idx]) * 10 + int(s[idx + 1]) <= 26:
+                    cnt += self.rec(idx + 2, s, n)
+
+            self.memo[idx] = cnt
+
+        return self.memo[idx]
+
+    def numDecodings(self, s: str) -> int:
+
+        self.memo = [-1] * (len(s) + 1)
+        self.memo[len(s)] = 1
+
+        return self.rec(0, s, len(s))
+