109. Convert Sorted List to Binary Search Tree.cpp

/**
 * 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 == NULL)
            return NULL;
        ListNode *slow = head, *fast = head, *prev = NULL;
        while (fast->next != NULL && fast->next->next != NULL)
        {
            prev = slow;
            slow = slow->next;
            fast = fast->next->next;
        }
        TreeNode *root = new TreeNode(slow->val);
        if (prev == NULL)
        {
            head = NULL;
        }
        else
            prev->next = NULL;
        ListNode *nexxt = slow->next;
        slow->next = NULL;
        root->left = sortedListToBST(head);
        root->right = sortedListToBST(nexxt);
        return root;
    }
};