leetcode_convert_sorted_list_to_binary_search_tree

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难度：Medium

解题思路：采用分治的方法。在链表中找到中间节点，创建树节点，然后截断这个链表，分为两段。再分别调用函数。

代码如下：

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
/**
 * Definition for a binary tree node.
 * 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;
        if(head->next == NULL)
            return new TreeNode(head->val);
        ListNode *dummy = new ListNode(-1);
        dummy->next = head;
        ListNode *slow = dummy;
        ListNode *fast = dummy;
        while(fast->next != NULL && fast->next->next != NULL)
        {
            slow = slow->next;
            fast = fast->next->next;
        }
        fast = slow->next;
        slow->next = NULL;
        TreeNode *new_node = new TreeNode(fast->val);
        new_node->left = sortedListToBST(head);
        new_node->right = sortedListToBST(fast->next);
        return new_node;
    }
};

运行结果：22ms，超过85.76％