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.

 

class Solution {

    public TreeNode sortedListToBST(ListNode head) {

        if (head == null)

            return null;

        return toBST(head, null);

    }

    

    private TreeNode toBST(ListNode head, ListNode tail) {

        ListNode slow = head;

        ListNode fast = head;

        if (head == tail)

            return null;

        while (fast != tail && fast.next != tail) {

            fast = fast.next.next;

            slow = slow.next;

        }

        //例如链表为1 2 3 4 5 6 7 8，则slow指向5，fast指向null，5作为thead， 

        TreeNode thead = new TreeNode(slow.val);

        thead.left = toBST(head, slow);

        thead.right = toBST(slow.next, tail);

        return thead;

    }

    

}