109. Convert Sorted List to Binary Search Tree

/*Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.BST:Binary search tree 平衡二叉查找树平衡二叉查找树定义为每个节点的两个子树高度差不超过1.     若左子树不为空，则左子树上的所有节点的值均小于它的根节点的值 若右子树不为空，则右子树上的所有节点的值均大于它的根节点的值 左、右子树也分别为二叉排序树二叉查找树是插入，排序，查找的平均时间复杂度都是O(Log(n))，最差情况是O(n),二叉树的平衡因子是该结点的左子树的深度减去它的右子树的深度，平衡二叉树的BF因子绝对为1。 *//** * Definition for singly-linked list. * struct ListNode { *     int val; *     struct ListNode *next; * }; *//** * Definition for a binary tree node. * struct TreeNode { *     int val; *     struct TreeNode *left; *     struct TreeNode *right; * }; */struct TreeNode* sortedListToBSTr(struct ListNode *head,struct ListNode *tail){if(head==tail)return NULL;if(head->next == tail){struct TreeNode *root = (struct TreeNode*)malloc(sizeof(struct TreeNode));root->val=head->val;root->left=NULL;root->right=NULL;return root;}struct ListNode *fast=head,*mid=head;while(fast!=tail && fast->next!=tail)//快慢指针寻找中间节点{mid=mid->next;fast=fast->next->next;}struct TreeNode *root = (struct TreeNode*)malloc(sizeof(struct TreeNode));root->val=mid->val;root->left=sortedListToBSTr(head,mid);root->right=sortedListToBSTr(mid->next,tail);return root;}struct TreeNode* sortedListToBST(struct ListNode* head) {    return sortedListToBSTr(head,NULL);}