【leetcode】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.

class Solution {
public:
int listlen(ListNode * node)
{
int len=0;
while(node)
{
++len;
node=node->next;
}
return len;
}

TreeNode * createBST(ListNode *node,int left,int right)
{
if(left>right) return NULL;

int mid=(left+right)/2;

ListNode *p=node;

for(int i=left;i<mid;i++)
p=p->next;

TreeNode *leftNode=createBST(node,left,mid-1);
TreeNode *rightNode=createBST(p->next,mid+1,right);

TreeNode *root=new TreeNode(p->val);
root->left=leftNode;
root->right=rightNode;

return root;
}

TreeNode* sortedListToBST(ListNode* head) {
int len=listlen(head);
return createBST(head,0,len-1);
}
};