#293 Decode Ways

Given a string s containing only digits, return the number of ways it can be decoded according to:

'A' -> "1"
'B' -> "2"
...
'Z' -> "26"

Rules:

• Each digit or pair of digits must map to a valid letter (1-26)
• No leading zeros allowed in groups
• Multiple valid groupings are counted separately

Constraints:

• 1 ≤ s.length ≤ 100
• s contains only digits

Examples:

// Example 1:
console.log(numDecodings("12"));
// Output: 2
// Explanation: Two ways to decode:
// - 1,2 -> "AB"
// - 12 -> "L"

// Example 2:
console.log(numDecodings("01"));
// Output: 0
// Explanation: Leading zero makes this invalid