---
tags: leetcode
---

# [109. Convert Sorted List to Binary Search Tree](https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree/)
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# My Solution
## The Key Idea for Solving This Coding Question
## C++ Code

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */


class Solution {
public:
    TreeNode *sortedListToBST(ListNode *head) {
        if (head == nullptr) {
            return nullptr;
        }
        if (head->next == nullptr) {
            return new TreeNode(head->val);
        }

        ListNode *preSlow = nullptr, *slow = head, *fast = head;
        while (fast != nullptr && fast->next != nullptr) {
            preSlow = slow;
            slow = slow->next;
            fast = fast->next->next;
        }

        ListNode *head2 = slow->next;
        slow->next = nullptr;
        preSlow->next = nullptr;

        TreeNode *node = new TreeNode(slow->val);
        node->left = sortedListToBST(head);
        node->right = sortedListToBST(head2);

        return node;
    }
};

## Time Complexity
$O(nlogn)$
* $n$ is the number of nodes in the singly linked list referenced by `head`.
## Space Complexity
$O(n)$
# Miscellaneous
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# Test Cases

[-10,-3,0,5,9]

[]

[0]

[1,3]

[1,2,3]

[-2,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]

-->