feat: update lc problems

[yanglbme]

No description provided.

[Copilot]

Pull request overview

This PR updates several LeetCode solution READMEs to add/expand “Solution 1” explanations (algorithm name + short rationale) and improves a few formatting inconsistencies in examples.

Changes:

• Add named solution headings (e.g., “Array”, “Mathematics”, “Greedy + Priority Queue”) and richer algorithm explanations.
• Expand step-by-step descriptions and complexity notes across multiple problems.
• Minor formatting cleanup in example blocks (whitespace consistency).

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
solution/0600-0699/0641.Design Circular Deque/README_EN.md	Adds an “Array” approach explanation and complexity notes
solution/0600-0699/0641.Design Circular Deque/README.md	Expands the Chinese explanation for the array-based circular deque
solution/0600-0699/0640.Solve the Equation/README_EN.md	Adds a “Mathematics” solution summary and related-problem link
solution/0600-0699/0630.Course Schedule III/README_EN.md	Cleans example formatting and adds greedy + max-heap explanation
solution/0600-0699/0609.Find Duplicate File in System/README_EN.md	Adds “Hash Table” solution explanation and complexity
solution/0600-0699/0609.Find Duplicate File in System/README.md	Expands Chinese hash-table explanation and complexity wording
solution/0600-0699/0602.Friend Requests II Who Has the Most Friends/README_EN.md	Fixes example whitespace and adds “Union All + Group By” explanation
solution/0600-0699/0602.Friend Requests II Who Has the Most Friends/README.md	Adds a short Chinese explanation for merge + group approach

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[Copilot]

The direction of the front pointer updates appears reversed for a typical circular deque where front points to the current front element. Usually, insertFront decrements front (wrap-around), while deleteFront increments front. Please adjust the wording (or add an explicit formula like front = (front - 1 + capacity) % capacity / front = (front + 1) % capacity) so the explanation matches the actual index movement semantics.

Suggested change

	When $\textit{insertFront}$ is called, we first check if the queue is full; if so, return $\text{false}$. Otherwise, we move $\textit{front}$ one position forward (using modular arithmetic for circular wrapping), insert the new element at $\textit{front}$, and increment $\textit{size}$ by 1.
	When $\textit{insertLast}$ is called, we first check if the queue is full; if so, return $\text{false}$. Otherwise, we compute the insertion position (using $\textit{front}$ and $\textit{size}$), insert the new element there, and increment $\textit{size}$ by 1.
	When $\textit{deleteFront}$ is called, we first check if the queue is empty; if so, return $\text{false}$. Otherwise, we move $\textit{front}$ one position backward (using modular arithmetic for circular wrapping) and decrement $\textit{size}$ by 1.
	When $\textit{insertFront}$ is called, we first check if the queue is full; if so, return $\text{false}$. Otherwise, we move $\textit{front}$ one position backward (for example, $\textit{front} = (\textit{front} - 1 + \textit{capacity}) \bmod \textit{capacity}$), insert the new element at $\textit{front}$, and increment $\textit{size}$ by 1.
	When $\textit{insertLast}$ is called, we first check if the queue is full; if so, return $\text{false}$. Otherwise, we compute the insertion position (using $\textit{front}$ and $\textit{size}$), insert the new element there, and increment $\textit{size}$ by 1.
	When $\textit{deleteFront}$ is called, we first check if the queue is empty; if so, return $\text{false}$. Otherwise, we move $\textit{front}$ one position forward (for example, $\textit{front} = (\textit{front} + 1) \bmod \textit{capacity}$) and decrement $\textit{size}$ by 1.

[Copilot]

The direction of the front pointer updates appears reversed for a typical circular deque where front points to the current front element. Usually, insertFront decrements front (wrap-around), while deleteFront increments front. Please adjust the wording (or add an explicit formula like front = (front - 1 + capacity) % capacity / front = (front + 1) % capacity) so the explanation matches the actual index movement semantics.

Suggested change

	When $\textit{insertFront}$ is called, we first check if the queue is full; if so, return $\text{false}$. Otherwise, we move $\textit{front}$ one position forward (using modular arithmetic for circular wrapping), insert the new element at $\textit{front}$, and increment $\textit{size}$ by 1.
	When $\textit{insertLast}$ is called, we first check if the queue is full; if so, return $\text{false}$. Otherwise, we compute the insertion position (using $\textit{front}$ and $\textit{size}$), insert the new element there, and increment $\textit{size}$ by 1.
	When $\textit{deleteFront}$ is called, we first check if the queue is empty; if so, return $\text{false}$. Otherwise, we move $\textit{front}$ one position backward (using modular arithmetic for circular wrapping) and decrement $\textit{size}$ by 1.
	When $\textit{insertFront}$ is called, we first check if the queue is full; if so, return $\text{false}$. Otherwise, we move $\textit{front}$ one position backward (e.g., $\textit{front} = (\textit{front} - 1 + \textit{capacity}) \bmod \textit{capacity}$), insert the new element at $\textit{front}$, and increment $\textit{size}$ by 1.
	When $\textit{insertLast}$ is called, we first check if the queue is full; if so, return $\text{false}$. Otherwise, we compute the insertion position (using $\textit{front}$ and $\textit{size}$), insert the new element there, and increment $\textit{size}$ by 1.
	When $\textit{deleteFront}$ is called, we first check if the queue is empty; if so, return $\text{false}$. Otherwise, we move $\textit{front}$ one position forward (e.g., $\textit{front} = (\textit{front} + 1) \bmod \textit{capacity}$) and decrement $\textit{size}$ by 1.

[Copilot]

insertFront / insertLast 的描述里使用了“如果队列不为空”，这与前面的“是否已满”判断不一致（队列为空时也应允许插入）。建议将该条件改为“否则”（即不满即可插入），并同时用明确的下标更新公式说明 front 的移动方向（避免与实现语义产生冲突）。