Add decode remove bits

Author: agpeshal

dynamic_programming/counting_bits.py

@@ -0,0 +1,50 @@
+'''
+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.
+'''
+
+def num_decodings(s: str):
+    '''
+    >> num_decodings("12")
+    2
+    >> num_decodings("226")
+    3
+    '''
+    if not s or int(s[0]) == 0:
+        return 0
+    
+    last = 1
+    second_last = 1
+    
+    for i in range(1, len(s)):
+        
+        # 0 is a special digit since it does not
+        # correspond to any alphabet but can be
+        # meaningful if preceeded by 1 or 2
+        
+        if s[i] == "0":
+            if s[i-1] in {"1", "2"}:
+                curr = second_last
+            else:
+                return 0
+            
+        elif 11 <= int(s[i-1:i+1]) <= 26:
+            curr = second_last + last
+        else:
+            curr = last
+        
+        last, second_last = curr, last
+
+    return last
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()