109. Convert Sorted List to Binary Search Tree

#code

class Solution {
public:
    
    TreeNode* sortedListToBST(ListNode* head) {
        if(head == NULL) return NULL;
        return constructBST(head,NULL);
    }
    
    TreeNode* constructBST(ListNode* head,ListNode* tail){
        if(head == tail)
    		return NULL;
    	if(head->next == tail){	
    		TreeNode *root = new TreeNode( head->val );
    		return root;
    	}
    	ListNode* mid = head;
        ListNode* second = head;
    	while(second!= tail && second->next != tail) {
    		mid = mid->next;
    		second = second->next->next;
    	}
        TreeNode* node = new TreeNode(mid->val);
        node -> left = constructBST(head,mid);
        node -> right = constructBST(mid->next,tail);
        return node;
    }
   
};