Leetcode 109: Convert Sorted List to Binary Search Tree

Posted 2020-10-14 Keep walking

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

 

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     public int val;
 5  *     public ListNode next;
 6  *     public ListNode(int x) { val = x; }
 7  * }
 8  */
 9 /**
10  * Definition for a binary tree node.
11  * public class TreeNode {
12  *     public int val;
13  *     public TreeNode left;
14  *     public TreeNode right;
15  *     public TreeNode(int x) { val = x; }
16  * }
17  */
18 public class Solution {
19     public TreeNode SortedListToBST(ListNode head) {
20         var list = new List<int>();
21         var p = head;
22         
23         while (p != null)
24         {
25             list.Add(p.val);
26             p = p.next;
27         }
28         
29         int[] nums = list.ToArray();
30         
31         return DFS(nums, 0, nums.Length - 1);
32     }
33     
34     private TreeNode DFS(int[] nums, int start, int end)
35     {
36         if (start > end) return null;
37         if (start == end) return new TreeNode(nums[start]);
38         
39         int mid = start + (end - start) / 2;
40         var root = new TreeNode(nums[mid]);
41         root.left = DFS(nums, start, mid - 1);
42         root.right = DFS(nums, mid + 1, end);
43         
44         return root;
45     }
46 }