Update euler-0038.cpp

[honeyranjan1]

Summary of Changes:

This pull request corrects the logic in the original euler-0038.cpp, which aimed to solve a variation of Project Euler Problem 38 but had multiple issues including incorrect digit handling and output strategy.

Key Fixes & Improvements:

Corrected the concatenation logic:
Replaced fragile integer-based digit construction with reliable std::string concatenation to properly form the full number sequence.

Replaced bitmask with boolean array:
Simplified digit tracking using a boolean array for readability and clarity, especially when checking for uniqueness and avoiding the digit zero.

Handled variable digit length (K):
Generalized the solution to handle both 8-digit and 9-digit pandigital requirements as per the input K.

Loop structure redesigned:
Ensured correct and complete concatenated products are tested by iterating multipliers until the combined string length matches K.

Output logic changed:
Instead of printing only the largest pandigital number, the code now outputs all valid multipliers in ascending order as per the modified problem statement.

🏷️ Added descriptive header comment block with title, description, input/output format, and author attribution.