[Leetcode] Convert Sorted List to Binary Search Tree

题目：

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

思路：类似二分法，中间向左右两边找child.

class Solution {
public:
    TreeNode* buildBST_helper(ListNode* head, int length) {
        if (head == nullptr || length <= 0) return nullptr;
        int index = (length - 1) / 2;
        ListNode* mid = head;
        for (int i = 0; i < index; ++i) {
            mid = mid->next;
        }
        TreeNode* new_root = new TreeNode(mid->val);
        new_root->left = buildBST_helper(head, index);
        new_root->right = buildBST_helper(mid->next, length - index - 1);
        return new_root;
    }
    
    TreeNode *sortedListToBST(ListNode *head) {
        int length = 0;
        ListNode* runner = head;
        while (runner != nullptr) {
            length++;
            runner = runner->next;
        }
        return buildBST_helper(head, length);
    }
};

总结：复杂度为O(n).