/*
 * @lc app=leetcode id=109 lang=java
 *
 * [109] Convert Sorted List to Binary Search Tree
 */

// @lc code=start
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode sortedListToBST(ListNode head) {
        return buildTree(head);   
    }

    private TreeNode buildTree(ListNode head) {
        if(head == null || head.next == null) {
            return head == null ? null : new TreeNode(head.val, null, null);
        }   
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode slow = dummy, fast = dummy, prev = dummy;
        while(fast != null && fast.next != null) {
            fast = fast.next.next;
            prev = slow;
            slow = slow.next;
        }
        prev.next = null;
        TreeNode tree = new TreeNode(slow.val, null, null);
        tree.left = buildTree(dummy.next);
        tree.right = buildTree(slow.next);
        return tree;
    }
}
// @lc code=end