Check if all leaf nodes are at same level in scala

Scala program for Check if all leaf nodes are at same level. Here problem description and other solutions.

/* 
  Scala program for
  Check if all leaves are at same level 
*/
// Binary Tree Node
class TreeNode(var data: Int,
	var left: TreeNode,
		var right: TreeNode)
{
	def this(data: Int)
	{
		// Set node value
		this(data, null, null);
	}
}
class BinaryTree(var root: TreeNode,
	var depth: Int)
{
	def this()
	{
		this(null, -1);
	}
	def checkLeafHeight(node: TreeNode, level: Int): Unit = {
		if (node != null)
		{
			// Test node is leaf or not
			if (node.left == null && node.right == null)
			{
				if (this.depth == -1)
				{
					// This is first leaf node
					// Get node level
					this.depth = level;
				}
				else if (this.depth != level)
				{
					// When two leaf nodes is different level
					this.depth = -2;
					return;
				}
			}
			if (this.depth != -2)
			{
				// Recursively visit left and right subtree
				checkLeafHeight(node.left, level + 1);
				checkLeafHeight(node.right, level + 1);
			}
		}
	}
	def isSameLevelLeaf(): Unit = {
		this.depth = -1;
		// Test
		this.checkLeafHeight(this.root, 1);
		// Display result
		if (this.depth < 0)
		{
			// Two possible case
			// 1) When tree empty
			// 2) When leaf node are not same level
			println("No");
		}
		else
		{
			// When leaf node is same level
			println("Yes");
		}
	}
}
object Main
{
	def main(args: Array[String]): Unit = {
		// Create new tree
		var tree: BinaryTree = new BinaryTree();
		/*   
		    Make A Binary Tree
		    --------------------
		      1
		     /  \
		    2    3
		   /    /  \
		  4    5    6
		       
		*/
		//insertion of binary Tree nodes
		tree.root = new TreeNode(1);
		tree.root.left = new TreeNode(2);
		tree.root.right = new TreeNode(3);
		tree.root.right.right = new TreeNode(6);
		tree.root.right.left = new TreeNode(5);
		tree.root.left.left = new TreeNode(4);
		tree.isSameLevelLeaf();
		// Add new node
		tree.root.left.left.left = new TreeNode(7);
		/*
		   When add new node
		    -----------------------
		          1
		         /  \
		        2    3
		       /    /  \
		      4    5    6
		     /
		    7  
		*/
		tree.isSameLevelLeaf();
	}
}

Output

Yes
No