/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
class Solution {public TreeNode sortedListToBST(ListNode head) {if(head==null)return null;ListNode l1=head,l2=head;int count=0;while(l1!=null){l1=l1.next;count++;  //得到链表的长度
        }// for(int i=0;i<count/2;i++){//     l2=l2.next;        //得到链表的中点// }return buildBST(head,0,count-1);}public TreeNode buildBST(ListNode head,int l,int r){if(l>r)return null;int mid=(l+r)/2;ListNode tem=head;for(int i=0;i<mid;i++)tem=tem.next;    //每次递归都要遍历链表TreeNode root=new TreeNode(tem.val);root.left=buildBST(head,l,mid-1);root.right=buildBST(head,mid+1,r);return root;}
}

class Solution {public TreeNode sortedListToBST(ListNode head) {if(head==null)return null;int count=0;ListNode l1=head;while(l1!=null){l1=l1.next;count++;}int[] nums=new int[count];for(int i=0;i<count;i++){nums[i]=head.val;head=head.next;        //转化为数组
        }return buildBST(nums,0,count-1);        //将排序数组转为二叉探索树
        }public TreeNode buildBST(int[] nums,int l,int r){if(l>r)return null; int mid=(l+r)/2;TreeNode root=new TreeNode(nums[mid]);root.left=buildBST(nums,l,mid-1);root.right=buildBST(nums,mid+1,r);return root;}
}

class Solution {public TreeNode sortedListToBST(ListNode head) {
//注意若子树只有两个节点，只需以首节点为根构造右子节点为其后节点的子树if(head==null)return null;if(head.next==null)return new TreeNode(head.val);ListNode fast=head,slow=head,last=slow;while(fast.next!=null&&fast.next.next!=null){last=slow;  //这里执行到最后一步的时候，last只比slow慢一个指针。slow=slow.next;fast=fast.next.next;  }TreeNode root=new TreeNode(slow.val);fast=slow.next;//fast部分的链表转化为右子树if(slow!=last){  last.next=null;root.left=sortedListToBST(head);}root.right=sortedListToBST(fast);return root;}
}