Convert Sorted List to Binary Search Tree

题目：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.

解：

/** * 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 *createBST(ListNode *start, ListNode * end){//主要思想：使用快慢指针实现查找中间节点的操作，然后将中间节点作为根节点即可        TreeNode *root;        if(start==end){            root=new TreeNode(start->val);            return root;        }        ListNode *pre=NULL;        ListNode *s=start;        ListNode *q=start;        while(q!=end && q->next && q->next!=end){            pre=s;            s=s->next;            q=q->next->next;        }        root=new TreeNode(s->val);        if(pre)            root->left=createBST(start, pre);        if(s->next)            root->right=createBST(s->next, end);        return root;    }    TreeNode* sortedListToBST(ListNode* head) {        if(head==NULL){            return NULL;        }        ListNode *end=head;        while(end->next){            end=end->next;        }        TreeNode *root=createBST(head, end);        return root;    }};