lintcode,排序列表转换为二分查找树

/**
* Definition for ListNode.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int val) {
*         this.val = val;
*         this.next = null;
*     }
* }
* Definition of TreeNode:
* public class TreeNode {
*     public int val;
*     public TreeNode left, right;
*     public TreeNode(int val) {
*         this.val = val;
*         this.left = this.right = null;
*     }
* }
*/
public class Solution {
/**
* @param head: The first node of linked list.
* @return: a tree node
*/
public TreeNode sortedListToBST(ListNode head) {
if(head == null) return null;
if(head.next == null) return new TreeNode(head.val);
int len = 0;
TreeNode root = null;
ListNode tmp = head;
while(tmp != null) {
tmp = tmp.next;
len++;
}
if(len == 2){
root = new TreeNode(head.val);
root.right = new TreeNode(head.next.val);
}else{
len = len/2;
ListNode listnode = head;
for(int i = 0; i < len; i++){
listnode = listnode.next;
}
ListNode pre =head;
while(pre.next != listnode){
pre = pre.next;
}
pre.next = null;
root = new TreeNode(listnode.val);
listnode = listnode.next;
root.left = sortedListToBST(head);
root.right = sortedListToBST(listnode);
}
return root;
}
}