feat: [Week 02-5] solve decode-ways

decode-ways/Chaedie.py

@@ -0,0 +1,57 @@
+"""
+dp 문제는 아직 어색해서 직접 풀진 못했습니다.
+"""
+
+"""
+재귀와 dp hash_map 을 활용한 캐싱 전략
+Time: O(n)
+Space: O(n) = O(n) + O(n)
+"""
+
+
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        dp = {len(s): 1}
+
+        def dfs(i):
+            if i in dp:
+                return dp[i]
+            if s[i] == "0":
+                return 0
+
+            # 한자리 숫자
+            res = dfs(i + 1)
+
+            # 두자리 숫자
+            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)
+
+
+"""
+iterative dp
+Time: O(n)
+Space: O(n)
+"""
+
+
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        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]