[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:
int length(ListNode *p){
int cnt = 0;
while(p){
cnt ++;
p = p -> next;
}
return cnt;
}
TreeNode *generate(ListNode *node,int left,int right){
if(left > right) return NULL;
int mid = (left + right) / 2;
ListNode *p = node;
for(int i = left;i < mid;i ++)
p = p -> next;
TreeNode *tleft = generate(node,left,mid - 1);
TreeNode *tright = generate(p -> next,mid + 1,right);
TreeNode *root = new TreeNode(p -> val);
root -> left = tleft;
root -> right = tright;
return root;
}
TreeNode *sortedListToBST(ListNode *head) {
int len = length(head);
return generate(head,0,len - 1);
}
};