ConvertSortedListToBinarySearchTree.java

package binary_search_tree;

/**
 * Description: https://leetcode.com/problems/convert-sorted-list-to-binary-search-tree
 * Difficulty: Medium
 * Time complexity: O(nlog n)
 * Space complexity: O(log n)
 */
public class ConvertSortedListToBinarySearchTree {

    public TreeNode sortedListToBST(ListNode head) {
        if (head == null) return null;
        if (head.next == null) return new TreeNode(head.val);

        ListNode mid = findMid(head);
        TreeNode root = new TreeNode(mid.val);
        root.left = sortedListToBST(head);
        root.right = sortedListToBST(mid.next);

        return root;
    }

    private ListNode findMid(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        ListNode prev = null;

        while (fast != null && fast.next != null) {
            prev = slow;
            slow = slow.next;
            fast = fast.next.next;
        }

        prev.next = null;
        return slow;
    }

    private static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        public TreeNode(int val) {
            this.val = val;
        }
    }

    private static class ListNode {
        int val;
        ListNode next;
    }
}