CODE

/**
* Definition for a binary tree node.
* public class TreeNode {
*     public int val;
*     public TreeNode left;
*     public TreeNode right;
*     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
*         this.val = val;
*         this.left = left;
*         this.right = right;
*     }
* }
*/

public class Solution {
  int diameter = 0;
  int curDiam = -1;

  public int DiameterOfBinaryTree(TreeNode root) {

    if (root == null)
    {
      if (curDiam > diameter)
        diameter = curDiam;
      return diameter;
    }
    curDiam++;
    if (root.left == null && root.right == null)
    {
      if (curDiam > diameter)
        diameter = curDiam;
      curDiam = 0;
    }
    DiameterOfBinaryTree(root.left);
    DiameterOfBinaryTree(root.right);
    curDiam++;

    return diameter;
    
  }
}