[LC] feat: 240825 decode ways

Author: HaJunYoo

decode-ways/hajunyoo.py

@@ -0,0 +1,21 @@
+class Solution:
+    # time complexity: O(n)
+    # space complexity: O(n)
+    def numDecodings(self, s: str) -> int:
+        if not s or s.startswith('0'):
+            return 0
+
+        n = len(s)
+        dp = [0] * (n + 1)
+        dp[0] = 1
+        dp[1] = 1 if s[0] != '0' else 0
+
+        for i in range(2, n + 1):
+            if s[i - 1] != '0':
+                dp[i] += dp[i - 1]
+
+            two_digit = int(s[i - 2:i])
+            if 10 <= two_digit <= 26:
+                dp[i] += dp[i - 2]
+
+        return dp[n]