Decode Ways

Decode Ways is a Medium Data Structures and Algorithms interview problem you can solve, run and submit on Thita.ai. It belongs to the Dynamic Programming (DP) Patterns pattern, in the 1D Array (Fibonacci Style) subpattern.

Problem statement

You have intercepted a secret message encoded as a string of numbers. The message is decoded via the following mapping:

"1" -> 'A'

"2" -> 'B'

...

"25" -> 'Y'

"26" -> 'Z'

However, while decoding the message, you realize that there are many different ways you can decode the message because some codes are contained in other codes ("2" and "5" vs "25").

For example, "11106" can be decoded into:

- "AAJF" with the grouping (1, 1, 10, 6)

- "KJF" with the grouping (11, 10, 6)

- The grouping (1, 11, 06) is invalid because "06" is not a valid code (only "6" is valid).

Note: there may be strings that are impossible to decode.

Given a string s containing only digits, return the number of ways to decode it. If the entire string cannot be decoded in any valid way, return 0.

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).

Topics and companies

Topics: String, Dynamic Programming.

Reported in interviews at Adobe, Amazon, Apple, ByteDance, Cisco, Databricks, Facebook, Goldman Sachs, Google, JPMorgan, Lyft, Microsoft and others.

How to practise Decode Ways on Thita.ai

Starter code is provided in C, C#, C++, Go, Java, JavaScript and Python. Your solution runs against 5 test cases for this problem, with the AI coach available for a hint when you are stuck rather than a finished answer. The reference solution runs in O(n) time and O(1) space.

Open Decode Ways in the code editor, or read the Decode Ways editorial for a worked solution with its approach and complexity analysis.

Where Decode Ways sits in the DSA pattern sheet

Problems related to Decode Ways

Other problems that use the same 1D Array (Fibonacci Style) and Dynamic Programming (DP) Patterns techniques:

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