[LeetCode] 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.

[Solution].java

/**
 * 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 {
    public TreeNode sortedListToBST(ListNode head) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int len = 0;
        ListNode run = head;
        while(run != null){
            len++;
            run = run.next;
        }
        ListNode[] ref = new ListNode[1];
        ref[0] = head;
        return bottomUp(ref, 1, len);
    }

    public TreeNode bottomUp(ListNode[] ref, int begin, int end){
        if(begin > end) return null;
        int mid = (begin + end) / 2;
        TreeNode left = bottomUp(ref, begin, mid - 1);
        TreeNode parent = new TreeNode(ref[0].val);
        parent.left = left;
        ref[0] = ref[0].next;
        parent.right = bottomUp(ref, mid + 1, end);
        return parent;
    }
}