Decrypt String from Alphabet to Integer Mapping

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.

The test cases are generated so that a unique mapping will always exist.

LeetCode Problem - 1309

class Solution {
    // Method to decode a string 's' into its corresponding alphabet characters
    public String freqAlphabets(String s) {
        StringBuilder sb = new StringBuilder();  // StringBuilder to store decoded characters

        // Iterate through the string 's' from the end to the beginning
        for (int i = s.length() - 1; i >= 0; i--) {
            int k = 0;  // Variable to store numeric value

            // Check if current character is '#'
            if (s.charAt(i) == '#') {
                // Extract the numeric value from the substring (last two characters before '#')
                k = Integer.parseInt(s.substring(i - 2, i));
                i -= 2;  // Move index back by 2 to skip the processed substring
            } else {
                // Convert the current character to its integer value
                char ch = s.charAt(i);
                int integerValue = ch - '0';  // Convert character to integer value
                k = integerValue;  // Assign integer value to 'k'
            }

            k += 96;  // Adjust 'k' to ASCII value of corresponding alphabet characters (a = 97)
            sb.append((char) k);  // Append decoded character to StringBuilder
        }

        // Reverse the StringBuilder to get the decoded string in correct order
        return sb.reverse().toString();
    }
}