feat: cpp memory improve

[QuBenhao]

Correctly handle memory leaks or free twice in generated code.
Some of cases are commented, let user decide.

[Copilot]

Pull Request Overview

This PR improves memory management across generated solution code by replacing heap-allocated sentinels with stack-based dummies, adding delete calls to free allocated structures, and introducing destructors in model classes to prevent leaks or double‐frees.

• Use stack‐allocated dummy nodes in list‐building functions to avoid manual cleanup.
• Add explicit delete calls after converting solution results to JSON.
• Define destructors in model classes (TreeNodeNext, NodeRandom, NodeNeighbors) for automated cleanup.

Reviewed Changes

Copilot reviewed 27 out of 27 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
problems/problems_236/Solution.cpp	Added delete root after JSON conversion
problems/problems_23/Solution.cpp	Switched dummy list nodes to stack allocation and added free
problems/problems_2/Solution.cpp	Switched dummy list nodes to stack allocation and added free
problems/problems_19/Solution.cpp	Added node deletion in removeNthFromEnd and free head
problems/problems_1656/Solution.cpp	Changed make_shared to make_unique for OrderedStream
problems/problems_160/Solution.cpp	Added cleanup logic for linked‐list cycle handling
problems/problems_146/Solution.cpp	Added ~LRUCache destructor and switched to make_unique
problems/problems_142/Solution.cpp	Added cycle break and head deletion for detectCycle
problems/problems_141/Solution.cpp	Added cycle break and head deletion for hasCycle
problems/problems_138/Solution.cpp	Added deletion of original and copied random lists
problems/problems_1379/Solution.cpp	Added deletion of original and cloned trees
problems/problems_114/Solution.cpp	Added delete root after flattening
problems/problems_105/Solution.cpp	Added delete res_ptr after building tree
daily-problems.json	Updated "daily" value
cpp/models/TreeNodeNext.h	Added destructor to delete left and right
cpp/models/NodeRandom.h	Added destructor to delete next
cpp/models/NodeNeighbors.h	Added destructor to delete neighbor pointers
cpp/models/NodeNeighbors.cpp	Changed parameter to reference and initialized vector slots
cpp/models/ListNode.h	Changed functions to use const refs and return by value
cpp/models/ListNode.cpp	Switched list builders to stack‐allocated dummies

Comments suppressed due to low confidence (1)

cpp/models/ListNode.h:26

• Changing the signature from non-const to const& and returning by raw pointer may break existing callers. Ensure this API change is acceptable or version it accordingly.

ListNode *IntArrayToListNode(const std::vector<int> &arr);