Leetcode 109. Convert Sorted List to Binary Search Tree

[medium]

Convert Sorted List to Binary Search Tree - LeetCode

Given the head of a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example 1:

Input: head = [-10,-3,0,5,9]
Output: [0,-3,9,-10,null,5]
Explanation: One possible answer is [0,-3,9,-10,null,5], which represents the shown height balanced BST.

Example 2:

Input: head = []
Output: []

Example 3:

Input: head = [0]
Output: [0]

Example 4:

Input: head = [1,3]
Output: [3,1]

Constraints:

• The number of nodes in head is in the range [0, 2 * 104].
• -10^5 <= Node.val <= 10^5

[Think]

1. a sorted list want to get a BST- will need to find the head of the tree first
2. how to find the head in a list, use fast, slow pointer
3. use recursion for left subtree and right subtree

[Note]

1. have to check fast != tail first then fast.next != tail, or it will overflow

[Second — use different approach to the BST