LeetCode Convert Sorted List to Binary Search Tree

题目：

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 cnt = 0;
ListNode *cur = head;
while(cur != NULL) {
cnt++;
cur= cur->next;
}
TreeNode *root = create(head, 0, cnt-1);
return root;
}
private:
//自底向上构造树，左边构造完时，创建root，然后构造右子树
TreeNode *create(ListNode *&head, int begin, int end) {
if(begin > end)
return NULL;
int mid = begin + (end-begin)/2;
TreeNode *left = create(head, begin, mid-1);
TreeNode *root = new TreeNode(head->val);
root->left = left;
head = head->next;
TreeNode *right = create(head, mid+1, end);
root->right = right;
return root;
}
};