[leetcode 109] 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) {
        if (!head) {
            return NULL;
        }
        ListNode *end = head;
        while (end->next) {
            end = end->next;
        }
        return build(head, end);
        
    }
    TreeNode *build(ListNode *begin, ListNode *end) {
        if (!begin){
            return NULL;
        }
        if (!begin->next) {
            TreeNode *root = new TreeNode(begin->val);
            return root;
        }
        ListNode dummy(-1);
        dummy.next = begin;
        auto slow = &dummy;
        auto fast = begin;
        while (fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        TreeNode *root = new TreeNode(slow->next->val);
        auto head = slow->next->next;
        slow->next = NULL;
        root->left = build(begin, slow);
        root->right = build(head, end);
        return root;
        
    }
};