feat solve decode-ways

Author: lylaminju

decode-ways/pmjuu.py

@@ -0,0 +1,31 @@
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        if s[0] == "0":
+            return 0
+
+        prev, curr = 1, 1
+
+        for i in range(1, len(s)):
+            temp = curr
+
+            if s[i] == "0":
+                if s[i - 1] in ("1", "2"):
+                    curr = prev
+                else:
+                    return 0
+            else:
+                two_num = int(s[i - 1] + s[i])
+                is_two_num_decoded = 10 <= two_num <= 26
+                if is_two_num_decoded:
+                    curr += prev
+            
+            prev = temp
+
+        return curr
+
+
+# Time Complexity: O(n)
+# - The loop iterates through the string once, where n is the length of the string.
+
+# Space Complexity: O(1)
+# - Only two variables (prev and curr) are used, independent of the input size.