Dec 27

Author: GSNCodes

December_LeetCode_Challenge/Decode_Ways.py

@@ -0,0 +1,60 @@
+A message containing letters from A-Z is being encoded to numbers using the following mapping:
+
+'A' -> 1
+'B' -> 2
+...
+'Z' -> 26
+Given a non-empty string containing only digits, determine the total number of ways to decode it.
+
+The answer is guaranteed to fit in a 32-bit integer.
+
+ 
+Example 1:
+Input: s = "12"
+Output: 2
+Explanation: It could be decoded as "AB" (1 2) or "L" (12).
+
+Example 2:
+Input: s = "226"
+Output: 3
+Explanation: It could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).
+
+Example 3:
+Input: s = "0"
+Output: 0
+Explanation: There is no character that is mapped to a number starting with '0'. 
+We cannot ignore a zero when we face it while decoding. So, each '0' should be part of "10" --> 'J' or "20" --> 'T'.
+
+Example 4:
+Input: s = "1"
+Output: 1
+ 
+Constraints:
+
+1 <= s.length <= 100
+s contains only digits and may contain leading zero(s).
+
+
+
+
+# O(n) Time and O(n) Space
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        
+        dp = [0 for i in range(len(s)+1)]
+        dp[0] = 1
+        dp[1] = 0 if s[0]=='0' else 1
+        
+        for i in range(2, len(s)+1):
+            
+            one_digit = s[i-1:i]
+            two_digits = s[i-2:i]
+            
+            if int(one_digit) >= 1:
+                dp[i] += dp[i-1]
+            
+            if 10 <= int(two_digits) <= 26:
+                dp[i] += dp[i-2]
+            
+        
+        return dp[-1]