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 binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution 
{
public:
    TreeNode *sortedListToBST(ListNode *head) 
    {
        int size=0;
        ListNode* p=head;
        while(p!=NULL) 
        {
            size++;
            p=p->next;
        }
        return generate(head,size);
    }
    
    TreeNode* generate(ListNode* head,int size)
    {
        if(head==NULL || size==0) return NULL;
        
        int index=(size+1)/2;
        ListNode* p=head;
        for(int i=1;i<index;i++) p=p->next;
        TreeNode* root=new TreeNode(p->val);
        
        root->left=generate(head,index-1);
        root->right=generate(p->next,size-index);
        
        return root;
    }
};