[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 a binary tree node. * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    /*alorithm: divide and conquere    two pointer    1->2->3->4->5 //f:3,s = 5    1->2->3->4 //f:3 , s = 4    */    TreeNode* sortedListToBST(ListNode* head) {            ListNode* first = head,*second = head;            ListNode* ftail = NULL;            while(second&&second->next){                second = second->next;                if(second->next)second = second->next;                ftail = first;                first = first->next;            }            TreeNode* root = NULL;            if(first){                root = new TreeNode(first->val);                if(ftail)ftail->next = NULL;                root->left = ftail == NULL?NULL:sortedListToBST(head);                root->right = sortedListToBST(first->next);            }            return root;    }};

class Solution {public:    int getCount(ListNode* head){        int count = 0;        while(head){            count++;            head = head->next;        }        return count;    }    //[start,end)    TreeNode* sortedListToBSTSub(ListNode* &head,int start,int end){            if(start >= end)return NULL;            int m = start + (end-start)/2;            TreeNode* l,*r,*root;            l = sortedListToBSTSub(head,start,m);            root = new TreeNode(head->val);            head = head->next;            r = sortedListToBSTSub(head,m+1,end);            root->left = l;            root->right = r;            return root;    }    TreeNode* sortedListToBST(ListNode* head) {        int n = getCount(head);        return sortedListToBSTSub(head,0,n);    }};