CODE 24: 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.

	public TreeNode sortedListToBST(ListNode head) {
		// Start typing your Java solution below
		// DO NOT write main() function
		return dfs(head);
	}

	TreeNode dfs(ListNode head) {
		TreeNode node = null;

		if (head != null) {
			ListNode perMidNode = null;
			ListNode midNode = head;
			ListNode lastNode = head;
			while (null != lastNode.next && null != lastNode.next.next) {
				perMidNode = midNode;
				midNode = midNode.next;
				lastNode = lastNode.next.next;
			}
			if (null != perMidNode) {
				perMidNode.next = null;
			}
			node = new TreeNode(midNode.val);
			TreeNode left = dfs(perMidNode == null ? null : head);
			TreeNode right = dfs(midNode.next);
			node.left = left;
			node.right = right;
		}
		return node;
	}