Add LeetCode Problems and Solutions

Add problems and solutions from [LeetCode](https://leetcode.com/) following the points stated in [Contributing.md](https://github.com/soh-sarkar/Coding-Questions/blob/main/CONTRIBUTING.md).

Read CONTRIBUTING.md first to identify the required format and contribution points, then inspect the repository's existing problem and solution entries. Choose a LeetCode problem that fits those guidelines, add its problem and solution in the established structure, and verify that the submission matches the documented requirements.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.