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Decode the Coding Interview in Elixir: Real-World Examples

Feature #10: Decode a Message

Explore how to reverse a message digest function by applying arithmetic operations with given integers. Understand how to implement a depth-first search to test all combinations of operations and values. This lesson helps you develop skills to solve interview questions involving recursion, arithmetic operations, and backtracking in Elixir.

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Description

In this feature, we will reverse a message digest function. The message digest function applies a subset of the operators +, -, *, and / along with some parenthesization to individual digits in the message and converts the subset into the message digest. The message will consist of four integers, each in the range [1, 9]. The message digest is also an integer.

Note: It is also possible that no combination of these operations may lead to a message digest.

We will be given the array of integers and the message digest. We will be restricted by the following rules:

  • The division operator / represents real division, not integer division.

  • We cannot use - as a unary operator. Every operation is carried out between two numbers.

  • We cannot concatenate numbers together. For example, if message = ...