feat: add solutions for lc No.3831

[yanglbme]

No description provided.

[github-actions]

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Filename	Before	After	Improvement	Visual comparison
solution/3800-3899/3831.Median of a Binary Search Tree Level/images/screenshot-2026-01-27-at-20801pm.png	53.1 KB	6.8 KB	87.2%	View diff
solution/3800-3899/3831.Median of a Binary Search Tree Level/images/screenshot-2026-01-27-at-20926pm.png	22.8 KB	3.5 KB	84.6%	View diff

[Copilot]

Pull request overview

This PR adds LeetCode problem 3831 "Median of a Binary Search Tree Level" to the solution index files. The problem is a locked (premium) medium-difficulty problem.

Changes:

• Added problem 3831 entry to the English solution index
• Added problem 3831 entry to the Chinese solution index
• Minor formatting adjustment to a comment line in the main README

Reviewed changes

Copilot reviewed 10 out of 14 changed files in this pull request and generated 1 comment.

File	Description
solution/README_EN.md	Added entry for problem 3831 at line 3842 with correct formatting, link, and metadata (Medium difficulty, locked problem)
solution/README.md	Added entry for problem 3831 at line 3844 using English title (consistent with locked problems without Chinese translation)
README.md	Added line number prefix to a comment line (unrelated formatting change)

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[Copilot]

This change appears to be unrelated to adding solutions for LeetCode problem 3831. The modification only adds a line number prefix to a comment line, which seems to be a formatting adjustment rather than a functional change. This type of incidental formatting change should ideally be in a separate commit or PR to keep the changes focused and clear.

Suggested change

<!-- 排序算法、待补充 -->