如何判断一棵二叉树是完全二叉树

public class TreeNode
{
    public TreeNode(int v)
	{
	    Val = v;
	}
	
	public int Val { get; private set;}
	public TreeNode Left { get; set; }
	public TreeNode Right { get; set; }
}

public bool IsCompleteBinaryTree(TreeNode root)
{
    if(root == null) return true;
	
    Queue<TreeNode> queue = new Queue<TreeNode>();
	queue.Enqueue(root);
	
	bool shouldBeLeaf = false;
	while(queue.Count > 0)
	{
	        var node = queue.Dequeue();
		if(shouldBeLeaf && (node.Left != null || node.Right != null))
		{
		    return false;
		}
		
		if(node.Left == null && node.Right != null)
		{
		    return false;
		}
			
		if(node.Left != null)
		{
		    queue.Enqueue(node.Left);
		}
		
		if(node.Right != null)
		{
		    queue.Enqueue(node.Right);
		} 
		else 
		{
		    shouldBeLeaf = true;
		}
	}
		
    return true;
}