Add solutions for Combination Sum and Expression Add Operators

[ravithakur2k]

Added CombinationSum.py with recursive and iterative backtracking solutions for the Combination Sum problem. Added ExpressionAddOperators.py with two recursive solutions for generating expressions that evaluate to a target value, including optimizations with backtracking.

[super30admin]

Strengths:

• The student demonstrates a strong understanding of backtracking and recursion.
• The code is clean, well-commented, and easy to follow.
• The student provides multiple approaches (recursive and iterative) for the Combination Sum problem, showing versatility.

Areas for Improvement:

• In the CombinationSum.py file, the student has defined two Solution classes, which is not standard practice. It would be better to have one class with both methods or separate the solutions into different files.
• For the Expression Add Operators problem, the student could consider adding more comments to explain the calc and tail variables, as their roles might not be immediately obvious to someone reading the code for the first time.
• The student could also consider adding more test cases or edge cases in the comments to demonstrate the robustness of the solution.