LeetCode:Convert Sorted List to Binary Search Tree

Convert Sorted List to Binary Search Tree
Total Accepted: 67959 
Total Submissions: 224265 
Difficulty: Medium
Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.
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(M) Convert Sorted Array to Binary Search Tree
code:

/**
 * 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) {
        return sortedListToBST(head, NULL);
    }
    
    TreeNode* sortedListToBST(ListNode* head, ListNode *tail) {
        
        if(head == tail) return NULL;
        
        ListNode *fast = head,*slow = head;
        while(fast!=tail && fast->next != tail){ //       
            fast = fast->next->next;
            slow = slow->next;
        }
        TreeNode *root = new TreeNode(slow->val);
        root->left = sortedListToBST(head, slow);
        root->right = sortedListToBST(slow->next, tail);
        return root;
    }
    
};