109. Convert Sorted List to Binary Search Tree

Given 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:

Given the sorted linked list: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5

TreeNode* helper(ListNode* start, ListNode* end) {
    if (start == end) return nullptr;
    ListNode *fast = start, *slow = start;
    while (fast->next != end && fast->next->next != end) {
        fast = fast->next->next;
        slow = slow->next;
    }
    TreeNode* res = new TreeNode(slow->val);
    res->left = helper(start, slow);
    res->right = helper(slow->next, end);
    return res;
}
TreeNode* sortedListToBST(ListNode* head) { // time: O(n); space: O(logn)
    if (!head) return nullptr;
    return helper(head, nullptr);
}