/**

 * 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) {

        if(!head) return NULL;

        TreeNode* treeHead = buildTree(head);

        return treeHead;

    }

    TreeNode* buildTree(ListNode *head)

    {

        if(!head) return NULL;

        if(!head->next) return new TreeNode(head->val);

        ListNode* dummy = new ListNode(0);

        dummy->next = head;

        ListNode* prev = dummy, *slow = dummy->next, *fast = dummy->next->next;

        while(fast && fast->next)

        {

            slow = slow->next;

            prev = prev->next;

            fast = fast->next->next;

        }

        TreeNode* cur = new TreeNode(slow->val);

        TreeNode* left, *right;  // second error, cannot initialize left in the if-else

        right = buildTree(slow->next);

        if(prev != dummy)      // first error

        {

            prev->next = NULL;

            left = buildTree(head);

        }

        else left = NULL;

        cur->left = left;

        cur->right = right;

        delete dummy;  // first error, can not delete dummy too early if prev equals dummy

        return cur;

    }

};