Create 91-Decode-ways.py

Author: neetcode-gh

91-Decode-ways.py

@@ -0,0 +1,30 @@
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        # Memoization
+        dp = { len(s) : 1 }
+        def dfs(i):
+            if i in dp:
+                return dp[i]
+            if s[i] == "0":
+                return 0
+            
+            res = dfs(i + 1)
+            if (i + 1 < len(s) and (s[i] == "1" or 
+                s[i] == "2" and s[i + 1] in "0123456")):
+                res += dfs(i + 2)
+            dp[i] = res
+            return res
+        return dfs(0)
+
+        # Dynamic Programming
+        dp = { len(s) : 1 }
+        for i in range(len(s) - 1, -1, -1):
+            if s[i] == "0":
+                dp[i] = 0
+            else:
+                dp[i] = dp[i + 1]
+
+            if (i + 1 < len(s) and (s[i] == "1" or 
+                 s[i] == "2" and s[i + 1] in "0123456")):
+                dp[i] += dp[i + 2]
+        return dp[0]