#291 is completed with a recursive + memoization approach

Author: Hamid Gasmi

09-problems/lc_91_decode_ways.py

@@ -0,0 +1,35 @@
+
+class SolutionRecursive:
+    """
+    Intuition:
+        - Backtracking + memoization
+    
+    Complexity Analysis:
+        Time Complexity: O(N)
+        Space Complexity: O(N)
+    
+    """
+    def num_decodings(self, s: str) -> int:
+
+        memo = {}
+        return self.backtrack(s, 0, memo)
+
+    def backtrack(self, s: str, idx: int, memo: dict) -> int:
+        
+        len_s = len(s)
+        if idx == len_s:
+            return 1
+        
+        elif s[idx] == '0':
+            return 0
+
+        elif idx in memo:
+            return memo[idx]        
+                
+        decoding_count = self.backtrack(s, idx + 1, memo)
+        if idx + 1 < len_s and int(s[idx: idx + 2]) < 27:
+            decoding_count += self.backtrack(s, idx + 2, memo)
+        
+        memo[idx] = decoding_count
+        
+        return decoding_count