Leetcode#109 Convert Sorted List to Binary Search Tree

跟Convert Sorted Array to Binary Search Tree（参见这篇文章）类似，只不过用list就不能随机访问了。

代码：

 1 TreeNode *buildBST(ListNode *head, int len) {
 2   if (len <= 0)
 3     return NULL;
 4 
 5   ListNode *p = head;
 6   int leftLen = 1;
 7   while (leftLen * 2 < len) {
 8     p = p->next;
 9     leftLen++;
10   }
11   TreeNode *node = new TreeNode(p->val);
12   node->left = buildBST(head, leftLen - 1);
13   node->right = buildBST(p->next, len - leftLen);
14 
15   return node;
16 }
17 
18 TreeNode *sortedListToBST(ListNode *head) {
19   ListNode *node = head;
20   int len = 0;
21 
22   while (node) {
23     node = node->next;
24     len++;
25   }
26   return buildBST(head, len);
27 }