/**

  * Definition for a binary tree node.

  * public class TreeNode {

  *     int val;

  *     TreeNode left;

  *     TreeNode right;

  *     TreeNode(int x) { val = x; }

  * }

  */

 public class Solution {

     int maxNum = 0;

     public int diameterOfBinaryTree(TreeNode root) {

         int n = 0;

         if (root == null || (root.left == null && root.right == null)) {

             return 0;

         }

         helper(root);

         return maxNum;

     }

     public int helper(TreeNode root) {

         if (root == null) {

             return 0;

         }

         int left = helper(root.left);

         int right = helper(root.right);

         if (left + right > maxNum) {

             maxNum = left + right;

         }

         return left > right ? left + 1 : right + 1;

     }

 }