Leetcode 270: Closest Binary Search Tree Value

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     public int val;
 5  *     public TreeNode left;
 6  *     public TreeNode right;
 7  *     public TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public int ClosestValue(TreeNode root, double target) {
12         TreeNode candidate = null;
13         DFS(root, target, ref candidate);
14         return candidate.val;
15     }
16     
17     private void DFS(TreeNode node, double target, ref TreeNode candidate)
18     {
19         if (node == null) return;
20         
21         if (candidate == null || (Math.Abs((double)node.val - target) < Math.Abs((double)candidate.val - target))) 
22         {
23             candidate = node;
24         }
25         
26         if ((double)node.val > target)
27         {
28             DFS(node.left, target, ref candidate);
29         }
30         else
31         {
32             DFS(node.right, target, ref candidate);
33         }
34     }
35 }