Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longestpath between any two nodes in a tree. This path may or may not pass through the root.

Example:
Given a binary tree

          1
         /         2   3
       / \     
      4   5    

Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].

Note: The length of path between two nodes is represented by the number of edges between them.

 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 DiameterOfBinaryTree(TreeNode root) {
12         int curMax = 0;
13         
14         DFS(root, ref curMax);
15         
16         return curMax > 0 ? curMax - 1
 
 : 0;
17     }
18     
19     private int DFS(TreeNode node, ref int curMax)
20     {
21         if (node == null) return 0;
22         
23         int left = DFS(node.left, ref curMax);
24         int right = DFS(node.right, ref curMax);
25         
26         curMax = Math.Max(curMax, left + right + 1);
27         return Math.Max(left + 1, right + 1);
28     }
29 }

Leetcode 543: Diameter of Binary Tree