Leetcode - Convert Sorted List to Binary Search Tree

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.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; next = null; }
 * }
 */
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    static ListNode head;
    public TreeNode sortedListToBST(ListNode head) {
        if(head == null)    return null;
        this.head = head;
        
        // get list length
        int len = 0;
        ListNode ln = head;
        while(ln != null){
            len++;
            ln = ln.next;
        }   
        
        return sortedListToBST(0, len - 1);
    }
    
    // build bottom-to-top
    public TreeNode sortedListToBST(int start, int end) {
        // if finished (root)
        if(start > end) return null;
        
        // get mid val
        int mid = (start + end) / 2;
        
        // build left sub tree
        TreeNode left = sortedListToBST(start, mid - 1);
        // build root node
        TreeNode root = new TreeNode(head.val);
        root.left = left;
        // move to next node to build right sub tree
        head = head.next;
        root.right = sortedListToBST(mid + 1, end);
        
        return root;
    }
}

Reference: http://leetcode.com/2010/11/convert-sorted-list-to-balanced-binary.html