Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST. http://leetcode.com/onlinejudge

leetcode_convert_sorted_list_to_binary_search_tree.cpp

/**
 * 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 get_list_num_(ListNode* head) {
        int ret = 0;
        while (head) {
            ret++;
            head = head->next;
        }
        return ret;
    }

    ListNode* get_list_mid_(ListNode* head, int n) {
        int mid = (n - 1) / 2;
        while (mid--)
            head = head->next;
        return head;
    }

    TreeNode* get_bst_(ListNode* head, int n) {
        if (n == 0) {
            return NULL;
        }
        ListNode* mid = get_list_mid_(head, n);
        int mid_idx = (n + 1) / 2;
        TreeNode* node = new TreeNode(mid->val);
        node->left = get_bst_(head, mid_idx - 1);
        node->right = get_bst_(mid->next, n - mid_idx);
        return node;
    }

    TreeNode *sortedListToBST(ListNode *head) {
        int n = get_list_num_(head);
        return get_bst_(head, n);
    }
};