【LeetCode】Convert Sorted List to Binary Search Tree

/**
* 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 getMiddleNode(ListNode head){
if(head == null || head.next == null) return head;
ListNode dummy = new ListNode(-1);
dummy.next = head;
ListNode fast = head,slow = head,pre = dummy;
while(fast.next != null && fast.next.next != null){
pre = slow;
slow = slow.next;
fast = fast.next.next;
}
pre.next = null;
return slow;
}
public TreeNode sortedListToBST(ListNode head) {
if(head == null )return null;
if(head.next == null) return new TreeNode(head.val);
ListNode middle = getMiddleNode(head);
if(head == middle) head = null;//防止栈溢出，当只有两个节点时候，middle=head节点，此时head应该置为null，表示左子树为空
TreeNode root = new TreeNode(middle.val);
root.left = sortedListToBST(head);
root.right = sortedListToBST(middle.next);
return root;
}
}