Add subset sum algorithm

[AliAlimohammadi]

Description

This PR implements the Subset Sum Problem using dynamic programming. The algorithm determines whether there exists a subset of a given array that sums to a specified target value.

The implementation uses a 2D DP table where dp[i][j] represents whether sum j can be achieved using the first i elements of the array. This is a classic dynamic programming problem with applications in resource allocation, partitioning, and combinatorial optimization.

• Implements subset sum problem using DP approach
• Includes comprehensive test cases with edge cases (empty arrays, zero sum, single elements)
• Time complexity: O(n * sum) where n is array length and sum is the target value
• Space complexity: O(n * sum) for the DP table

Type of change

• New feature (non-breaking change which adds functionality)

Checklist:

• I ran below commands using the latest version of rust nightly.

• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.

• I ran cargo fmt just before my last commit.

• I ran cargo test just before my last commit and all tests passed.

• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).

• I added my algorithm to DIRECTORY.md with the correct link.

• I checked CONTRIBUTING.md and my code follows its guidelines.

• Code structure follows repository patterns

• All tests are included and logically sound

• Documentation is comprehensive with examples

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.44%. Comparing base (13710a7) to head (a2fe724).

Additional details and impacted files

@@ Coverage Diff @@ ## master #956 +/- ## ========================================== + Coverage 95.39% 95.44% +0.05%  ========================================== Files 332 333 +1 Lines 21496 21524 +28 ========================================== + Hits 20506 20544 +38  + Misses 990 980 -10 

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