[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);
    }
};