记录贴，互勉

142. Linked List Cycle II
Find a cycle and the fast and slow meet point

23. Merge k Sorted Lists
Merge sort two list

21. Merge Two Sorted Lists
Merge two list

147. Insertion Sort List
1. Find the value greater than the next value
2. Find the position for the next value
3. Move the next value to the front
4. Update the next value

148. Sort List (merge sort)
1.Use two-pointer to break the list
2. merge
Hint: Break the list need to break the next pointer between this two

707. Design Linked List
maintain the size and head node

LinkedList

BST（二叉搜索树）

98. Validate Binary Search Tree
recursive solution (use stack to iterate)

98- Iterative solution