[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.

/**

 * 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 *DFS(vector<int> &arr,int left,int right)

    {

        if(right<left) return NULL;

        int mid=(right+left)>>1;

        //每次从表头开始遍历链表寻找中间结点超时

        TreeNode *root=new TreeNode(arr[mid]);

        root->left=DFS(arr,left,mid-1);

        root->right=DFS(arr,mid+1,right);

        return root;

    }

    TreeNode *sortedListToBST(ListNode *head) {

        ListNode *p=head;

        vector<int> arr;

        while(p)

        {

            arr.push_back(p->val);

            p=p->next;

        }

        return DFS(arr,0,arr.size()-1);

    }

};