Secret Code Shuffler

You are given a string where each pair of characters represents a letter and a shift value.

• The first character in each pair is a lowercase letter.
• The second character is a digit (0-9) representing how many positions to shift the letter in the ASCII character set.

Function Signature

function decodeSecretCode(s) {
  // Your code here
}

Parameters

• s: A string where characters are paired as (letter, shift)

Return Value

• Return the decoded string by applying the shifts to each letter.

Example

Input: s = "a2b3c1"
Output: "ced"
Explanation:
- "a2": 'a' (97) + 2 = 99 = 'c'
- "b3": 'b' (98) + 3 = 101 = 'e' 
- "c1": 'c' (99) + 1 = 100 = 'd'

Input: s = "x1y2z3"
Output: "y{}"
Explanation:
- "x1": 'x' (120) + 1 = 121 = 'y'
- "y2": 'y' (121) + 2 = 123 = '{'
- "z3": 'z' (122) + 3 = 125 = '}'

Input: s = "a0b0c0"
Output: "abc"
Explanation: Zero shifts leave letters unchanged

Rules

Character Pairs

• The string consists of pairs where:
  • First character: lowercase letter (a-z)
  • Second character: digit (0-9) representing shift amount

Shift Calculation

• Add the shift value to the letter's ASCII code.
• No wrapping - characters can go beyond 'z' to {, }, etc.
• Use charCodeAt() and fromCharCode() for conversion.

Invalid Inputs
Return empty string for:

• Empty or null strings
• Strings with odd length (incomplete pairs)
• Strings with non-letter characters in odd positions
• Strings with non-digit characters in even positions

Constraints

• 0 ≤ s.length ≤ 1000
• String length must be even (complete pairs)
• Only lowercase letters (a-z) in odd positions
• Only digits (0-9) in even positions

Algorithm

1. Check if input is valid (even length, proper character types).
2. Process string in pairs of two characters.
3. For each pair:
   • Extract letter and shift value.
   • Apply shift: newCharCode = letter.charCodeAt(0) + shift
   • Convert back to character: String.fromCharCode(newCharCode)
   • Add decoded character to result.

Test Cases

• Valid codes with various shift values
• Codes with characters beyond 'z' (e.g., {, }, |)
• Zero shift values
• Invalid inputs (odd length, wrong character types)
• Empty string
• Edge cases with maximum shifts