Find the Missing Digit – Coding Challenge Solution

Welcome! This repository contains a C# solution to the popular "Find the Missing Digit" coding challenge. Whether you’re prepping for interviews, leveling up your skills, or following along with our YouTube tutorial, you’re in the right place.

Check out the full walkthrough on YouTube that breaks down the challenge and shows you how to solve it.

About the Challenge

The “Find the Missing Digit” coding challenge is a classic problem often encountered in interviews, coding bootcamps, and online platforms like LeetCode or CodeSignal. It tests your ability to manipulate strings, arithmetic logic, and handle edge cases.

The Problem Statement

You are given a simple arithmetic equation where exactly one digit (0-9) has been replaced with a question mark (?). Your task is to determine which digit belongs in the place of ? so that the equation is valid.

Example:

Input: "1? + 23 = 35"
Output: 2 (because 12 + 23 = 35)

Input: "31 - ?2 = 150"
Output: -1 (because no digit can result in 150)

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Constraints

• Your code must handle the following operations:

  • Addition (+)
  • Subtraction (-)
  • Multiplication (*)
  • Division (/) (integer division only, no remainders, and never divide by zero)

• The ? can appear anywhere in the numbers on the left or right of the equation.

• There will only be one missing digit.

• Equations will always be provided as a string, and should be formatted as "{integer} {operator} {integer} = {integer}".

• Return -1 if it is impossible to create a mathematically correct equation or invalid input was received.

Edge Cases

• Division by zero must be avoided.
• Expressions with multiple ? should be rejected or handled appropriately (the provided code assumes only one ?).
• Ensure all numbers are valid integers after replacement.
• Reject answers that would result in numbers like 012 or 00 (not valid integers).

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What You Learn by Solving This

• String manipulation and parsing—critical for real-world data processing.
• Defensive coding—validating inputs, avoiding runtime errors, handling unexpected or malformed data.
• Understanding problem requirements—paying close attention to rules like “no leading zeros” or valid math.

Keep practicing challenges like this to build up the instincts and skills you need to excel in real-world coding and interviews!

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Try It Yourself

Clone the repo and run the solution:

git clone https://github.com/CoderFoundry/FindMissingDigit
cd FindMissingDigit

# run the application
dotnet run --project FindMissingDigit

# or run the included unit tests
dotnet test