[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.

跟上一题一样，只是数据结构换成了链表，这里求链表的中位数还是乖乖用len先计算长度吧，用slow、fast指针还得先标记尾指针，反而更麻烦。

/**
 * 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:
    void buildTree(TreeNode *&root, ListNode *head, int len) {
        if (len <= 0) return;
        ListNode *mid = head;
        for (int i = 0; i < len / 2; ++i) {
            mid = mid->next;
        }
        root = new TreeNode(mid->val);
        ListNode *head2 = mid->next;
        mid = NULL;
        buildTree(root->left, head, len / 2);
        buildTree(root->right, head2, (len - 1) / 2);
    }
    
    TreeNode *sortedListToBST(ListNode *head) {
        int len = 0;
        for (ListNode *p = head; p!= NULL; p = p->next) {
            ++len;
        }
        TreeNode *root = NULL;
        buildTree(root, head, len);
        return root;
    }
};