Check if all leaf nodes are at same level in c++

C++ program for Check if all leaf nodes are at same level. Here problem description and explanation.

// Include header file
#include <iostream>
using namespace std;
/*
  C++ program for
  Check if all leaves are at same level 
*/
// Binary Tree Node
class TreeNode
{
	public: 
    int data;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int data)
	{
		// Set node value
		this->data = data;
		this->left = nullptr;
		this->right = nullptr;
	}
};
class BinaryTree
{
	public: TreeNode *root;
	int depth;
	BinaryTree()
	{
		this->root = nullptr;
		this->depth = -1;
	}
	void checkLeafHeight(TreeNode *node, int level)
	{
		if (node != nullptr)
		{
			// Test node is leaf or not
			if (node->left == nullptr && 
                node->right == nullptr)
			{
				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
				this->checkLeafHeight(node->left, level + 1);
				this->checkLeafHeight(node->right, level + 1);
			}
		}
	}
	void isSameLevelLeaf()
	{
		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
			cout << "No" << endl;
		}
		else
		{
			// When leaf node is same level
			cout << "Yes" << endl;
		}
	}
};
int main()
{
	// Create new tree
	BinaryTree *tree = 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();
	return 0;
}

Output

Yes
No