91. Decode Ways Solution

Author: doh6077

decode-ways/doh6077.py

@@ -0,0 +1,20 @@
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        if s[0] == '0':
+            return 0
+        n = len(s)
+        cur = 1
+        pre1 = pre2 = 1
+        for i in range(2, n+1):
+            one = int(s[i-1])
+            two = int(s[i-2:i])
+            cur = 0
+            if 1 <= one <= 9:
+                cur += pre1
+            if 10 <= two <= 26:
+                cur += pre2
+
+            pre2 = pre1
+            pre1 = cur
+
+        return cur