# LeetCode — Decode ways (Blind Curated 24)

A message containing letters from `A-Z` can be encoded into numbers using the following mapping:

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

To decode an encoded message, all the digits must be grouped then mapped back into letters using the reverse of the mapping above (there may be multiple ways). For example, `"11106"` can be mapped into:

• `"AAJF"` with the grouping `(1 1 10 6)`
• `"KJF"` with the grouping `(11 10 6)`

Note that the grouping `(1 11 06)` is invalid because `"06"` cannot be mapped into `'F'` since `"6"` is different from `"06"`.

Given a string `s` containing only digits, return the number of ways to decode it.

The test cases are generated so that the answer fits in a 32-bit integer.

Example 1:

`Input: s = "12"Output: 2Explanation: "12" could be decoded as "AB" (1 2) or "L" (12).`

Example 2:

`Input: s = "226"Output: 3Explanation: "226" could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).`

Example 3:

`Input: s = "06"Output: 0Explanation: "06" cannot be mapped to "F" because of the leading zero ("6" is different from "06").`

Constraints:

• `1 <= s.length <= 100`
• `s` contains only digits and may contain leading zero(s).

Solution:

Time complexity: O(N)

Space complexity: O(N)

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