Decrypt String from Alphabet to Integer Mapping

Problem Description

You are given a string s formed by digits and '#'. We want to map s to English lowercase characters as follows:

• Characters ('a' to 'i') are represented by ('1' to '9') respectively.
• Characters ('j' to 'z') are represented by ('10#' to '26#') respectively.

Return the string formed after mapping.

Key Insights

• The mapping is unique due to the constraints.
• We need to differentiate between single digit mappings (1-9) and double digit mappings followed by '#' (10-26#).
• We can iterate through the string and build the result based on the presence of '#' to determine the character mapping.

Space and Time Complexity

Time Complexity: O(n), where n is the length of the input string. Space Complexity: O(1), since we are using a constant amount of extra space for the result string.

Solution

To solve the problem, we will iterate through the input string and check for the presence of '#' to determine if we are dealing with a single-digit or double-digit mapping. We will use a result list to build the final output string efficiently, and then convert it to a string before returning.

1. Initialize an empty list to store the characters.
2. Loop through the string using an index.
3. If the current character is followed by '#', convert the preceding two characters to the corresponding letter and add it to the result.
4. If the current character is not followed by '#', convert the single character to the corresponding letter and add it to the result.
5. Finally, return the joined list as the output string.

Code Solutions