LeetCode109 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. （Medium）

分析： 

链表不同于数组的是需要先遍历一遍求出长度，然后再找到中点，其余思路与Convert Sorted Array to Binary Search Tree一致。

代码：

/**
 * 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 {
private:
    TreeNode* helper(ListNode* head, int sz) {
        if (head == nullptr || sz <= 0) {
            return nullptr;
        }
        ListNode* runner = head;
        for (int i = 0; i < sz / 2; ++i) {
            runner = runner -> next;
        }
        TreeNode* root = new TreeNode(runner -> val);
        root -> left = helper(head, sz / 2);
        root -> right = helper(runner -> next, sz - sz / 2 - 1);
        return root;
    }
public:
    TreeNode* sortedListToBST(ListNode* head) {
        int sz = 0;
        ListNode* runner = head;
        while (runner != nullptr) {
            sz++;
            runner = runner -> next;
        }
        TreeNode* result = helper(head, sz);
        return result;
    }
};