solve: decode ways

Author: sounmind

decode-ways/evan.py

@@ -0,0 +1,24 @@
+def numDecodings(s: str) -> int:
+    if not s:
+        return 0
+
+    n = len(s)
+
+    # dp[i] represents the number of ways to decode the string s[:i]
+    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":
+            # If the one-digit number is valid, we can decode it
+            dp[i] += dp[i - 1]
+
+        two_digit = int(s[i - 2 : i])
+
+        if 10 <= two_digit <= 26:
+            # If the two-digit number is valid, we can decode it
+            dp[i] += dp[i - 2]
+
+    return dp[n]