feat: add LeetCode Hot 100 Medium problems collection

Added 22 high-frequency LeetCode Hot 100 Medium problems with detailed solutions:

**Core Problems:**
- 两数相加 (LeetCode 2) - Linked list manipulation
- 无重复字符的最长子串 (LeetCode 3) - Sliding window
- 三数之和 (LeetCode 15) - Sorting + two pointers

**Array & String:**
- 盛最多水的容器 (LeetCode 11) - Two pointers
- 最长回文子串 (LeetCode 5) - DP / expand around center
- 除自身以外数组的乘积 (LeetCode 238) - Prefix/suffix product
- 最长连续序列 (LeetCode 128) - HashSet

**Backtracking:**
- 电话号码的字母组合 (LeetCode 17) - Backtracking
- 括号生成 (LeetCode 22) - DFS
- 子集 (LeetCode 78) - Backtracking

**Linked List:**
- 删除链表的倒数第N个结点 (LeetCode 19) - Fast/slow pointers

**Binary Tree:**
- 二叉树的中序遍历 (LeetCode 94) - Recursive/iterative
- 二叉树的最大深度 (LeetCode 104) - DFS/BFS
- 翻转二叉树 (LeetCode 226) - Recursive
- 对称二叉树 (LeetCode 101) - Recursive
- 路径总和 (LeetCode 112) - DFS
- 从前序与中序遍历序列构造二叉树 (LeetCode 105) - Recursive

**Dynamic Programming:**
- 柱状图中最大的矩形 (LeetCode 84) - Monotonic stack
- 最大正方形 (LeetCode 221) - DP
- 完全平方数 (LeetCode 279) - DP

**Stack:**
- 最小栈 (LeetCode 155) - Stack design

**DFS/BFS:**
- 单词搜索 (LeetCode 79) - DFS backtracking

Each problem includes:
- Detailed problem description with examples
- Multiple solution approaches
- Go and Java implementations
- Time & space complexity analysis
- Advanced follow-up questions
- P7-level bonus points (deep understanding, real-world applications)

Total: 22 problems, 2500+ lines of content
Suitable for P7 backend interview preparation

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

.obsidian/workspace.json

@@ -196,6 +196,22 @@
   },
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   "lastOpenFiles": [
+    "16-LeetCode Hot 100/从前序与中序遍历序列构造二叉树.md",
+    "16-LeetCode Hot 100/路径总和.md",
+    "16-LeetCode Hot 100/对称二叉树.md",
+    "16-LeetCode Hot 100/翻转二叉树.md",
+    "16-LeetCode Hot 100/二叉树的最大深度.md",
+    "16-LeetCode Hot 100/二叉树的中序遍历.md",
+    "16-LeetCode Hot 100/最小栈.md",
+    "16-LeetCode Hot 100/除自身以外数组的乘积.md",
+    "16-LeetCode Hot 100/最长连续序列.md",
+    "16-LeetCode Hot 100/完全平方数.md",
+    "16-LeetCode Hot 100/最大正方形.md",
+    "16-LeetCode Hot 100/柱状图中最大的矩形.md",
+    "16-LeetCode Hot 100/单词搜索.md",
+    "16-LeetCode Hot 100/子集.md",
+    "16-LeetCode Hot 100/最长回文子串.md",
+    "16-LeetCode Hot 100/括号生成.md",
     "16-LeetCode Hot 100/删除链表的倒数第N个结点.md",
     "16-LeetCode Hot 100/电话号码的字母组合.md",
     "16-LeetCode Hot 100/盛最多水的容器.md",
@@ -208,22 +224,6 @@
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     "questions/05-并发编程/ConcurrentHashMap原理.md",
     "questions/02-数据库/MySQL主从延迟.md",
-    "questions/02-数据库/分库分表.md",
-    "questions/02-数据库/数据库锁机制.md",
-    "questions/02-数据库/事务隔离级别.md",
-    "questions/14-Web3与区块链/Golang与区块链开发.md",
-    "questions/02-数据库/MyBatis核心原理.md",
-    "questions/02-数据库/MySQL索引优化.md",
-    "questions/01-分布式系统/MySQL主从延迟.md",
-    "questions/01-分布式系统/CAP理论和BASE理论.md",
-    "questions/01-分布式系统/一致性哈希.md",
-    "questions/01-分布式系统/数据库锁机制.md",
-    "questions/01-分布式系统/数据库分库分表.md",
-    "questions/01-分布式系统/事务隔离级别.md",
-    "questions/01-分布式系统/分布式ID生成.md",
-    "questions/15-简历面试/场景设计题.md",
-    "questions/15-简历面试/个人发展题.md",
-    "questions/15-简历面试/离职原因与动机.md",
     "questions/15-简历面试",
     "questions/14-Web3与区块链",
     "12-面试技巧",