# 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: 2

Explanation: "12" could be decoded as "AB" (1 2) or "L" (12).

**Example 2:**

`Input: s = "226"`

Output: 3

Explanation: "226" could be decoded as "BZ" (2 26), "VF" (22 6), or "BBF" (2 2 6).

**Example 3:**

`Input: s = "06"`

Output: 0

Explanation: "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)**