[Documentation Update]: Add leet code solution for problem (18)

[Harleen-786]

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Issue Description

As a Gssoc contributor I want to add solution for problem 18 : "4Sum"

Suggested Change

I propose to add a comprehensive solution for Problem 19 on LeetCode, including the following sections:

1. Problem Description: A clear and concise restatement of the problem to ensure users understand the requirements.
2. Approach: A step-by-step explanation of the approach used to solve the problem, highlighting key concepts and logic.
3. Code Implementation: Well-commented code in the relevant programming language, demonstrating the solution.
4. Complexity Analysis: An analysis of the time and space complexity of the solution.

Rationale

No response

Urgency

High

Record

• I have read the Contributing Guidelines
• I'm a GSSOC'24 contributor
• I have starred the repository

[github-actions]

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[github-actions]

Hello @Harleen-786! Your issue #804 has been closed. Thank you for your contribution!