• 思路
  深度优先搜索

private:	
	int res = 0;

	int dfs(TreeNode* cur)
	{
		if(cur == nullptr)
		{
			return 0;
		}
		
		if(cur->left != nullptr)
		{
			int	leftSide = dfs(cur->left);
		}
		if(cur->right != nullptr)
		{
			int	rightSide = dfs(cur->right); 
		}
		
		res = max(res, leftNode + rightNode);//维护最大的二叉树直径
		
		return max(leftNode, rightNode) + 1;//返回该节点的最大深度
	}
	
	
public:
	int diameterOfBinaryTree(TreeNode* root) 
    {
		int res = INT_MIN;
		dfs(root);
		
		return res;
    }