Convert Sorted List to Binary Search Tree

原题是这个样子：

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

Thoughts

If you are given an array, the problem is quite straightforward. But things get a little more complicated when you have a singly linked list instead of an array. Now you no longer have random access to an element in O(1) time. Therefore, you need to create nodes bottom-up, and assign them to its parents. The bottom-up approach enables us to access the list in its order at the same time as creating nodes.

参考：http://www.programcreek.com/2013/01/leetcode-convert-sorted-list-to-binary-search-tree-java/

代码如下：

public TreeNode sortedListToBST(ListNode head) {if (head == null)return null;int len = 0;ListNode nextNode = head;while (nextNode != null) {nextNode = nextNode.next;len++;}return buildTree(head, 0, len - 1);}public TreeNode buildTree(ListNode head, int start, int end) {if (start > end)return null;int mid = (start + end) / 2;ListNode p = head;for (int i = start; i < mid; i++) {p = p.next;}TreeNode left = buildTree(head, start, mid - 1);TreeNode right = buildTree(p.next, mid + 1, end);TreeNode root = new TreeNode(p.val);root.left = left;root.right = right;return root;}