decode-ways solution

Author: i-mprovising

decode-ways/i-mprovising.py

@@ -0,0 +1,40 @@
+"""
+Time complexity O(n)
+Space complexity O(1)
+
+Dynamic programming
+"""
+
+
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        if s[0] == '0':
+            return 0
+        n = len(s)
+        if n == 1:
+            return 1
+        tmp = int(s[:2])
+        dp = [1, 1]
+        if 11 <= tmp <= 26:
+            if tmp != 20:
+                dp = [1, 2]
+        elif s[1] == '0':
+            if tmp not in [10, 20]:
+                return 0
+        if n == 2:
+            return dp[-1]
+        
+        for i in range(2, n):
+            if s[i] == '0':
+                if s[i-1] in ['1', '2']:
+                    cur = dp[0]
+                else:
+                    return 0
+            else:
+                cur = dp[1]
+                tmp = int(s[i-1:i+1])
+                if (11 <= tmp <= 26) and (tmp != 20):
+                    cur += dp[0]
+            dp = [dp[1], cur]
+
+        return dp[-1]