Printing binary tree by levels in java

Question

I want to know the code to print a binary tree level by level, I mean, if I have this tree:

I want to print it like: 5 3 2 4 6.

I know I need to do the tree depth method and I already did it, but I don't know what else to do.

Use a queue to enqueue the child node and pop out the next node to print. — nhahtdh
– nhahtdh, Commented Jul 2, 2012 at 2:49
@nhahtdh I can't use any other data structure rather than the binary tree — Johnny Dahdah
– Johnny Dahdah, Commented Jul 2, 2012 at 2:55
@alfasin No, I don't know what is BFS, how can I use that in this method? — Johnny Dahdah
– Johnny Dahdah, Commented Jul 2, 2012 at 2:56
If you'll google it you'll find thousands of implementations, for example: divms.uiowa.edu/~sriram/21/fall05/ExamplePrograms/ReaderFiles/… you can read more about BFS here: en.wikipedia.org/wiki/Breadth-first_search — Nir Alfasi
– Nir Alfasi, Commented Jul 2, 2012 at 2:57

Community · Accepted Answer · 2020-06-20 09:12:55Z

3

You can use the level traversal algorithm to print them.

The algorithm works as follows:

queue := < root >

while queue is not empty

 v := queue.front

 print v

 foreach s : s is a son of v

    queue.enqueue(s)

 queue.dequeue

edited Jun 20, 2020 at 9:12

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answered Jul 2, 2012 at 2:55

konjac

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6 Comments

Nir Alfasi Over a year ago

and by the way, push and pop are methods of stuck, not queue. For queue it's usually enqueue and dequeue`

konjac Over a year ago

@JohnnyDahdah why not? A queue is so easy to implement. There only need an array with two variables recording the head and the tail.

Johnny Dahdah Over a year ago

@KunHuang: I know but the teacher said "You cannot use any data structure rather than the binary tree you have"

konjac Over a year ago

@alfasin Thanks for your remind. I usually use C++, so I mix up the queue in STL and in common algorithm representation. But you also has a mistake: stack is spelt as stuck. :-)

konjac Over a year ago

@JohnnyDahdah eh, you can add a pointer field in every nodes of the binary tree to link the nodes as the output order.

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Nir Alfasi · Accepted Answer · 2012-07-02 03:30:28Z

1

Okay, I think I figured it out:
1. extend the class Node and add a property called height (int)
2. Calculate the height of each node of the tree (easy recursive function - no data-structure is needed)
3. use a for-loop, and run in-order traversal, for each height (level) and print the nodes of that level

answered Jul 2, 2012 at 3:30

Nir Alfasi

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Comments

Ahmed Derradji · Accepted Answer · 2022-04-28 12:38:54Z

0

public void displayByLevel(){
    ArrayList<Node> ar = new ArrayList<>();
    ar.add(root);
    displayByLevelHelper(ar);
}
private void displayByLevelHelper(ArrayList<Node> ar){
    if(ar.isEmpty()){
        return;
    }
    ArrayList<Node> nextAr= new ArrayList<>();
    for(Node n:ar){
        System.out.print(n.data + " ");
        if(n.left!=null){
            nextAr.add(n.left);
        }
        if(n.right!=null){
            nextAr.add(n.right);
        }
    }
    System.out.println();
    displayByLevelHelper(nextAr);
}

answered Apr 28, 2022 at 12:38

Ahmed Derradji

1

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