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.

本以为时间复杂度过不了，没想到竟然过了。

/**
* 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) {

return listToBSTUtil(head, NULL);
}

TreeNode *listToBSTUtil(ListNode *begin, ListNode *end) {
if (begin == NULL || begin == end) {
return NULL;
}
if (begin->next == end) {
return new TreeNode(begin->val);
}

ListNode *mid = begin;
ListNode *cur = begin;
while (cur != end && cur->next != end) {
mid = mid->next;
cur = cur->next->next;
}

TreeNode *root = new TreeNode(mid->val);
root->left = listToBSTUtil(begin, mid);
root->right = listToBSTUtil(mid->next, end);

return root;
}
};

======================第二次====================

/**
* 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 == NULL) {
return NULL;
}
else if (head->next == NULL) {
return new TreeNode(head->val);
}

ListNode *fast = head;
ListNode *slow = head;
ListNode *prev = NULL;
while (fast != NULL && fast->next != NULL) {
fast = fast->next->next;
prev = slow;
slow = slow->next;
}
prev->next = NULL;

TreeNode *root = new TreeNode(slow->val);
root->left = sortedListToBST(head);
root->right = sortedListToBST(slow->next);

return root;
}
};