Oct 2nd : Decode ways

Author: vishwajeet-hogale

Python/decodeWays.py

@@ -0,0 +1,46 @@
+"""
+    Leetcode 91. Decode ways 
+    Method used : Memoization 
+"""
+
+
+def check(s,mp): # Checks if the string is a valid number in the map of numbers
+    return int(s in mp.keys())
+g_d = dict()
+class Solution():
+    def helper(self,s,mp):
+        if(s in g_d):
+            return g_d[s]
+        if(s[0] == '0'): # if the string starts with 0, you cannot decode
+            return 0
+        if(len(s) == 2): # len(s) == 2; two cases : checking two characters(s[0] && s[1]) seperately and the s[0:2] 
+            return  check(s[0:2],mp) + self.helper(s[1:],mp)
+        if(len(s) == 1 ): 
+            return int(s in mp.keys())
+        """
+            There are two cases 
+            * Pick one character from the front 
+                * Check if the charcater chosen is valid
+            * Pick two character from the front
+                * Check if the charcater chosen is valid
+            the conditions below help to validate the conditions mentioned here.
+        """
+        if not check(s[0:2],mp) and check(s[0:1],mp):
+            g_d[s] = self.helper(s[1:],mp)
+            return g_d[s]
+        if not check(s[0:1],mp) and check(s[0:2],mp):
+            g_d[s] = self.helper(s[2:],mp)
+            return g_d[s]
+        g_d[s] = self.helper(s[1:],mp) + self.helper(s[2:],mp)
+
+        return g_d[s]
+    def numDecodings(self, s):
+        """
+        :type s: str
+        :rtype: int
+        """
+        mp = dict()
+        for i in range(1,27):
+            mp[str(i)] = chr(64+i)
+        # print(mp)
+        return self.helper(s,mp)