/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public TreeNode sortedListToBST(ListNode head) {
if(head == null) return null;
return helper(head, null);
}
public TreeNode helper(ListNode head, ListNode tail){
if(head == tail) return null;
//to find the root node
ListNode fast = head;
ListNode slow = head;
while(fast != tail && fast.next != tail){
fast = fast.next.next;
slow = slow.next;
}
TreeNode root = new TreeNode(slow.val);
root.left = helper(head, slow);
root.right = helper(slow.next, tail);
return root;
}
}
