排序列表转换为二分查找树

给出一个所有元素以升序排序的单链表，将它转换成一棵高度平衡的二分查找树

/** * Definition of ListNode * class ListNode { * public: *     int val; *     ListNode *next; *     ListNode(int val) { *         this->val = val; *         this->next = NULL; *     } * } * Definition of TreeNode: * class TreeNode { * public: *     int val; *     TreeNode *left, *right; *     TreeNode(int val) { *         this->val = val; *         this->left = this->right = NULL; *     } * } */class Solution {public:    /**     * @param head: The first node of linked list.     * @return: a tree node     */    TreeNode *sortedListToBST(ListNode *head) {        // write your code here        if (head == NULL)        {            return NULL;        }                int len = 0;        ListNode *p = head;        while (p != NULL)        {            len++;            p = p->next;        }                return createTree(head, 0, len-1);    }private:    TreeNode* createTree(ListNode *head, int left, int right)    {        if (left > right)        {            return NULL;        }        int mid = left + (right-left)/2;        ListNode *p = head;        for (int i = left; i < mid; i++)        {            p = p->next;        }                TreeNode *leftNode = createTree(head, left, mid-1);        TreeNode *rightNode = createTree(p->next, mid+1, right);                TreeNode *root = new TreeNode(p->val);        root->left = leftNode;        root->right = rightNode;                return root;    }};