Question

I tried to convert a List from 3{1{,2{,}},5{4{,},6{,}}}
to a Binary Tree like this

               3
           1       5
             2    4  6

I thought it would be easier to use recursion but I get stuck.

public void ListToTree (ArrayList al) {
    Iterator it = al.iterator(); 
    // n is the Tree's root
    BSTnode n = new BSTnode(it.next());
    recurse(al,it,n); 

}

void recurse (ArrayList al, Iterator it, BSTnode n) {
    if(!it.hasNext()) return;
    Object element = it.next();
    if(element=="{"){
            recurse(al,it,n.left());
            return;
    } else if (element==",") {
            recurse(al,it,n.right());
            return;
    } else if (element =="}") {

    }

}

I don’t know how to proceed and was wondering if it’s the right track. Please give me some hints how to solve it. Moreover, I realize I often get stuck on recursive questions. Is it because I always want to break it down? Should I just think top-down and double-check if it’s correct? Thanks in advance !

Answer 1

Firstly: are you bound to that terrible list representation? You can easily build a BST based on the BST rules with this code:

void insert(Node n, int value) {
     if(n == null) {
           n = new Node(value);
     } else if(value < n.value) {
           if(n.left == null) {
               n.left = new Node(value);
               return;
           }
           insert(n.left, value);
     } else if(value > n.value) {
           if(n.right == null) {
                n.right = new Node(value);
                return;
           }
           insert(n.right, value);
     }
}

You really don’t have to pass the iterator. Just use the values from the list. Also it is usually unadvised to use implementation types in method signatures. (i.e. ArrayList -> List).
Another big mistake here is that you don’t use == for value comparison, that is for reference comparison. Use equals instead, but you should downcast the Object after an instanceof test e.g.:

if( element instanceof String) {
    String seperator = (String)element;
    if("{".equals(separator))
         //do sth...

Btw the thing you are missing from the code is the actual insertion and the backwards navigation.
After you found the right subtree by navigating with the {-s and ,-s, check whether the element is an Integer then set it as a value for the current node. Backwards navigation should be in the } branch by either returning one level from the recusion and some tricks or calling the method on the parent of the actual node.

But I don’t suggest you to follow this direction, it is much easier to just use the values from the list and the simple insertion method.