Leetcode 109: 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.

 

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     public int val;
 *     public ListNode next;
 *     public ListNode(int x) { val = x; }
 * }
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode SortedListToBST(ListNode head) {
        var list = new List<int>();
        var p = head;
        
        while (p != null)
        {
            list.Add(p.val);
            p = p.next;
        }
        
        int[] nums = list.ToArray();
        
        return DFS(nums, 0, nums.Length - 1);
    }
    
    private TreeNode DFS(int[] nums, int start, int end)
    {
        if (start > end) return null;
        if (start == end) return new TreeNode(nums[start]);
        
        int mid = start + (end - start) / 2;
        var root = new TreeNode(nums[mid]);
        root.left = DFS(nums, start, mid - 1);
        root.right = DFS(nums, mid + 1, end);
        
        return root;
    }
}