试图在Java中的二叉搜索树上实现层序遍历

Question

Okay, so I'm trying to make a method that will return the level order of a basic binary search tree that carries int values in its nodes. I've figured out most of the other methods, such as insertion, post order and pre order, but I keep running into the same problem with the level order method

Here's the code:

private DoubleStackQueue<Node> queue = new DoubleStackQueue<Node>();
//this is a queue that uses two stacks, one for the front and one for the back.
//it works just like a queue.
public String levelOrder(){  
    s = "";  //The s is a private String that is implemented earlier
    queue.add(this);  
    while (!queue.isEmpty())  
    {  
        Node node = (Node)queue.remove();  
        if (!(node.equals(null))) {s += ""+node.getVal();}  
        if (!(left.equals(null))) {queue.add(node.left);}  
        if (!(right.equals(null))) {queue.add(node.right);}  
    }  
    return s;  
}

The main problem that I am having is that, when the program gets to a leaf node, it still adds its children to the queue, even though there are no children, just null, so I'll get a queue that has two nulls in front of the actual item in it. I originally had the if statements as (left,= null) and such. but that didn't work either. I'm just trying to figure out how to make the program recognize when there aren't any children? What do I need to do?

Answer 1

Within your code I can find the lines

if (!(left.equals(null))) {queue.add(node.left);}  
if (!(right.equals(null))) {queue.add(node.right);}

where in the condition it states left and right which are nowhere defined in the parts you show but on the right it reads node.left and node.right . Is this intentionally? I'd expect also node.left and node.right in the if conditions.

Answer 2

Look at this question: Level-order tree traversal I believe this is the same thing you're trying to achieve, is it not? This is a pretty classic problem, so in my experience it's been discussed over-and-over before.

Answer 3

Several comments:

1. The primary issue is that you're using left and right instead of node.left and node.right in your comparisons.

2. To compare against null use if (var != null) . Do not use equals() . If a variable is null you cannot call methods on it as that will trigger NullPointerException s.

3. Once you have fixed your code you will never have null inserted into the queue. The first object you add is this which is guaranteed to be non-null, and subsequently you always check for null before inserting items onto the queue. That means your first if (node != null) check is unnecessary.

4. The cast in (Node) queue.remove() should be unnecessary. Your compiler ought to warn you about this.

Result:

queue.add(this);

while (!queue.isEmpty())  
{  
    Node node = queue.remove();

    s += "" + node.getVal();

    if (node.left  != null) { queue.add(node.left);  }
    if (node.right != null) { queue.add(node.right); }
}