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.

最直接的思维是list赋值给array，O(n).

以下为bottom-up递归

/**
 * 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 ListNode cur;
	public TreeNode sortedListToBST(ListNode head) {
		// Start typing your Java solution below
		// DO NOT write main() function
		if(head == null)
			return null;
		int size = 1;
		ListNode ptr = head;
		this.cur = head;
		while(ptr.next != null){
			ptr = ptr.next;
			size++;
		}
		return convert(0, size - 1);
	}
	public TreeNode convert(int start, int end){
		if(start > end)
			return null;
		int mid = start + (end - start) / 2;
		TreeNode left = convert(start, mid - 1);
		TreeNode parent = new TreeNode(cur.val);
		parent.left = left;
		cur = cur.next;
		parent.right = convert(mid + 1, end);
		return parent;
	}
}