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.

/** * Definition for singly-linked list. * struct ListNode { *     int val; *     ListNode *next; *     ListNode(int x) : val(x), next(NULL) {} * }; *//** * Definition for a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    TreeNode* sortedListToBST(ListNode* head) {        if (head == NULL) return NULL;        if (head->next == NULL) return (new TreeNode(head->val));        int len = 0;        ListNode *cur = head;        while (cur) {            cur = cur->next;            ++len;        }        return sortedListToBST(head, len);    }    TreeNode* sortedListToBST(ListNode* head, int len) {        if (head == NULL || len == 0) return NULL;        int mid_index = len / 2;        int left_len = len / 2;        int right_len = len / 2 - 1 + len % 2;        ListNode *mid = head;        for (int i = 0; i < mid_index; ++i)            mid = mid->next;        ListNode *right = mid->next;        TreeNode *root = new TreeNode(mid->val);        if (left_len)        root->left = sortedListToBST(head, left_len);        if (right_len)        root->right = sortedListToBST(right, right_len);        return root;    }};